I will assume you are not trying to procure a large-scale multi-processor database server; if you are, I suggest you contact sales reps from HP, Sun, IBM and perhaps Fujitsu, who will trip over one another in an entertaining fashion as they try to sell you something.
For desktop systems for office use, it's worth asking yourself carefully whether you need anything more than the lowest-performance systems currently sold; indeed, consider how loud the fans on current fast computers are, and contemplate one of VIA's small, convenient, silent systems based around the Ezra chip.
If your interest is playing games, you probably want an Athlon 64 system, somewhere around the 3000+, with an nForce motherboard, DDR memory, and a Radeon 9600. Or a Pentium 4, 3GHz, with an i875 board, DDR memory, and a GeForce FX 5600. There's not much to choose between them.
If you tend to work with (use or write) programs whose source code you have, and you don't find yourself frequently waiting for the computer, small-form-factor, quiet machines make an office a much more pleasant place to be in.
If performance is critical, a Socket 939 Athlon64 or a 1-series Opteron is the way to go, though Pentium 4 systems are still somewhat cheaper for slightly lower performance. Be sure to use the manufacturer's libraries when they're available, they're better-optimised than the average user can manage with reasonable effort.
A non-obvious advantage of the P4 over the Athlon is Intel's program VTune, which makes looking for bottlenecks in programs an order of magnitude more straightforward - you can see source and assembler listings annotated with "number of cycles spent on this line", or "number of L1 cache misses on this line", or "number of unaligned accesses on this line" information - and thereby means you can use that sort of level of optimisation routinely, and early on in the development process. Yes, the Athlon has comparable performance counters, but VTune doesn't know how to talk to them; and this is a task where the user interface is surprisingly important.
For a number-crunching workstation or a cluster node, I'd recommend a dual-processor Opteron; the 244 and 246 are in a reasonable price range, though the motherboards which offer both dual CPUs and AGP graphics are not cheap.
If your tasks are too big for an Athlon 64 FX53, the first recommendation is that you try to find clever ways of shrinking the tasks, and the second that you try to make them run in parallel; otherwise, you're essentially doomed. It's no longer possible to get a single-processor system which is very much faster than the fastest computer you could buy at PC World.
Upgrades have become relatively unexciting ever since the advances gained from process improvements started dropping off, around the 1GHz Pentium 3, and it stopped being possible to get an enormous performance improvement without changing much of the infrastructure around the processor (and, therefore, the motherboard); I upgraded a P4/1300 to a P4/2400, but had a dual-channel Rambus motherboard which was overkill for the slower processor.
On the other hand, AMD have guaranteed that the Socket 940 infrastructure will be around for some time, and they will be offering dual-core Opteron chips in summer 2005. This gives a potential factor four or so of performance increase (between a dual 1400MHz and a dual dual-core 3GHz) on the same motherboard, which is getting quite interesting.
Buy an Opteron workstation. Or a G5 PowerMac. They're both good. They've both got decent memory bandwidth, attached directly to fast processors, with the two processors linked by a fast point-to-point bus. You won't get that much change from £2000 for either; if that's too much to pay, head to ebay and see what kind of dual-processor systems are available second-hand.
Dual Xeon systems are available for a bit less money, but the memory bandwidth per processor is noticably less than that of the Opterons, and, whilst the newest ones offer 64-bit processing, they do not appear to do as well as the Opterons at it. Dual Athlon MP systems are still just about available new, but I wouldn't bother.
The Killer Application for fast desktop systems still hasn't materialised; there's very little that can't be done with a lowest-end system, and memory quantity and disc speed are becoming more the restraining factors. I don't think I can justify non-entry-level machines as anything more than toys, except in processor-driven markets like 3D graphics and medical imaging - and even there I suspect there is more to be gained from optimising software than from hurtling towards the latest CPU from Intel or AMD. You almost start to believe that Moore's Law is slowing down.
With the Opteron and Athlon64 systems, AMD finally jumped back into the performance competition; if building a £1000 desktop nowadays, I'd base it around an Athlon64 3200+ chip, which seems comparable to a P4/3200 at similar price and has the additional 64-bit features waiting there for Microsoft to produce a 64-bit operating system. Whilst AMD's performance-rating scheme was looking a little dubious in the second half of 2002 and the start of 2003, it was recalibrated when the Athlon64 came out, and now makes more sense. And, since AMD now have a competitive product, they have lost their suicidal pricing scheme which at times meant their premium chips sold for significantly less than any Intel Pentium-branded processor; indeed, the higher-end Opterons are distressingly expensive.
AMD are in as good a competitive position as they've been for years, particularly with the disappointing performance of Prescott; they've even started making a profit, and their problem now is to ensure they can meet demand for the Athlon64. Prescott is not a success, the P4 Extreme Edition is too expensive to excite (its spectacular benchmark performance is more a matter of the L3 cache matching the working set of some of the SPEC applications). On the other hand, it has never made sense to get an AMD laptop; Intel's Banias, and even more so Dothan, offer an interesting combination of enormous performance and frugal power usage.
In the small-server market, I expect Opteron to make increasing inroads throughout 2004 and 2005, particularly in the four- and eight-processor market. The interesting battle will be Opteron vs Itanium 2: both chips are 64-bit, and the highest-end Madison processor is slightly slower than the Opteron 146 [which costs less than a tenth as much] at SPECint, though about 70% faster in SPECfp. A dual Opteron 246 is just about a reasonable machine for a well-off hobbyist to have at home, costing as much as a mediocre second-hand car; a dual Madison/6M would cost as much as a new SUV.