If you're looking for mainstream x86 chips then in terms of outright CPU/memory and IO performance then for the moment you're looking at Intel's Xeon 55xx-series chips (their new 75xx-series aren't out just yet and may be overkill for most people's needs anyway). I spend a lot of time choosing specific CPUs and the E5540 is a really nice price/power/heat/performance combo but there's no denying the X5570 and W5590 are the best performers, especially for low-thread-count apps often found in the Windows world due to their higher turbo-mode multipliers.
If you're looking for more of a balance between low cost and power-requirement/heat and care less about ultimate performance then AMD's Opteron 8435 or Intel's Xeon L5530 are good choices too. Unless price is the only concern I'd steer clear of desktop CPUs (i.e. Intel's 'Core' and 'i' range), there are many benchmarks that show them on equal parity with Xeon/Opterons but their 'drop-off' under heavy load is much more pronounced.
In terms of which servers to go for, I couldn't recommend HP's DL360/365 and DL380/385 models higher if you're looking for 2U rack-mount machines - although both IBM and Dell do very accomplished models in the same vein too.
As for which version of Windows, well I think you'd soon come to regret the Web edition, for the 2GB reason you state - memory is super cheap these days and helps enormously with pretty much every application out there - why limit yourself. At the other end of the scale it doesn't sound like you need the DC edition and Enterprise (although the version I exclusively buy) is really there for its clustering and 1:4 VM licensing capabilities - if you need that then go for Ent, if not then Standard will do you just fine.
By the way, you seem dead-set on going for 2003? 2008 is easily my favourite MS code ever, it's fast, easy to install, really stable, functional and has no EOL in sight - I'd implore you to consider it, especially in 64-bit guise.
Best of luck and feel free to come back with any follow-up questions.