Building a machine

[Erigion]

Processors:
If you're in for the long haul with this computer (2+ years) - it's recommended you get a 64-bit processor, your best deal for the money is either going to be an Athlon64 3200+ or Athlon64 3400+ (depending on if you want good, or biggest bang without busting your wallet - the AthlonFX is the bleeding edge, but will cost you almost 2x as much as the Athlon64).

If you're only planning to have this computer for 1-2 years - the Intel chips are ok, but in many cases Athlon64 3200+ will surpass them in the price : power ratio. (of course, Intel chips are still superior for rendering and office work, while Athlons are superior for games and such)

Current 64bit processors on the market: Optitron, AthlonFX, and Athlon64
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Video Cards:
I'd go with ATI Radeons, they're DX9.0 Compliant, nVidia suffer when using DX9.0 due to nVidia doing things it's own way :P.

Maybe watch and check to see if nVidia gets in gear with its newer cards (haven't kept on the ball with nVidia since the 5900 released)

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The Athlon 64 3000+ has the better price to performace ratio. The extra 512 KB of L2 Cache isn't really going to hamper performace. Neither the 3400+ or the 3200+ show enough gains to really justify spending the extra money on them. One problem with the Athlon64 chips is that the motherboard chipsets just aren't developed enough compared to the P4/XP chipsets. Another problem is that the AMD socket roadmap is a tad bit murky right now. So there is a chance that upgrading from a current socket 754 chip might prove difficult.

If you want to go with a 32 bit chip the ones you should consider are the P4 3.2C, P4 2.8C, or any of the Barton cores from AMD.

If you go Intel get a motherboard with the 875P chipset, if you go with an AthlonXP get a NForce2 motherboard, and the KT800 is the way to go for the Athlon64.

Bah, I'm two days late to tell you about the sale NewEgg had on a Radeon 9800 Pro, 230 for a nearly top of the line card is damn good. But if you want to spend 200 or less for a video card it's either between the Radeon 9600XT or the GeForce FX 5700 Ultra. If you're careful where you buy the 9600XT you'll still get the voucher for HL2, which is basically 50 dollars off that card. Another card to consider is the GeForce 5900SE which is just a bit above 200 dollar. Whatever you do, don't buy the 5600, that card was an utter failureand preformed horribly.

Like others have said, at the top it's between the 9800XT and the 5950Ultra, buying carefully will also get you a HL2 voucher for the 9800XT. Be prepared to spend at least 300 dollars for one of these cards, although both Nvidia and ATI are coming out with new cores in March, so these cards won't be at the top of the food chain soon.