A Gigabyte of RAM should be ample for most purposes. Only add more if you find you need it.

I'd suggest that the NVidia NForce3 250 chipset is the motherboard chipset of choice, rather than the VIA K8T800 that you've suggested.

I think Socket 754 is the realistic choice just yet and in the next few months. Socket 939 will accept more powerful processors in the future, but current Socket 939 motherboards for Athlon64 don't offer any better features at present than Socket 754 ones do, and by the time you change processors you'd probably want to upgrade to a better featured motherboard anyway. In upcoming months, NForce4 chipset, PCIE motherboards will become available for the platform, so if you aren't upgrading for a while yet that could become a realistic alternative.

Ditch the soundcard! The onboard audio with a decent motherboard will be just as good as the soundcard you've chosen.

Get a bigger hard drive. Games need lots of drive space lately, and that 80Gb would soon be filled.

The X800XT card might be claimed as the 'fastest' by Gaming magazines, but it's expensive and hard to obtain. The NVidia 6800GT is only very slightly slower, and is the best mix of price and performance currently available. Quite simply put, it's the best value gaming card on offer. That will help with balancing the budget.

And if you can swap that CD burner for a DVD burner, I'd reckon that's a better option. DVD burners will burn CDs as well, and if you use blank DVDs instead, you can back up your data with much less fuss and bother.

Those are my thoughts on the matter, anyway.

For a DVD Burner, I would recommend the NEC ND-3500A.

Have you given much thought to input devices? A fine keyboard/mouse combination would be the Logitech Cordless MX Duo. There are better keyboards and mice models available, but you can't beat this package for performance and cable-less usage.