Avoid anything AMD based, they are sorta confused at the moment and not really cheaper either. 2Q with their new chips might see a turnaround there, but all the "new builds" are going Core 2 Duo now.
The newer Core 2 Duo boards with the 1333FSB support should last you a good while, provided that everyone sticks with PCI Express for a while, which is seeming pretty likely. There is a move to DDR3 that is happening but RAM speed doesn't make a huge deal at all for performance on the current Intel architectures in the grand scheme of things. One of those boards should allow for an easy upgrade to a quad chip later on without any additional changes.
I'd wait at least to see what ATi comes up with later this month before buying anything if you can. I think, as a whole, the G80 line of cards is pretty underwhelming, period, save maybe the 8800GTS 320MB for people who don't need to run super-high resolutions. The R600 stuff might also suck, but we don't know that yet.
If you had to build something now that's budget-y, I'd get:
Core 2 Duo E4300
2GB DDR2 800
Decent motherboard, the Gigabyte DS3 is nice and is what I use.
ATi X1950Pro (or if you can swing it, an 8800GTS)
Overclock the CPU some and you are set to run games at pretty fancy settings (maybe not super-maxed) in DX9 at 1280x1024 or so.
Your upgrade path can be done totally piecemeal-you could add a cheap Core 2 Quad early next year with a clock speed boost, you could upgrade that video card to a DX10 part, and still keep everything else in your case.
The only thing you can't fudge on is the RAM. You need 2GB.
Also, if you are thinking of upgrading to fix a stuttering/rubberbanding issue with TQ, don't bother. It won't help, it's an engine defect that hopefully will get fixed in a patch.
