Go with the Opteron 144 over that AMD CPU, it's the same clock but doubled the L2 cache and OC's beautifully. This will raise your budget.
For the memory, stick with the higher-end PC4000 stuff, excellent for overclocking the Opteron series. This will raise your budget.
The XFX 6800XT owns any 6600GT, and is usually cheaper. This should lower your budget.
I don't think it's necessary to have a 250Gb HD unless you'll be keeping tons of media and multiple libraries of large 3D games, you could cut it down to a single 7200 120Gb HD and always add more space later. Also, I prefer Seagate over WD, but that's your choice. This should lower your budget.
With this setup, you will hold alot more OC potential and the budget will come in somewhere around your current planned setup. I would stress you pay the extra $25-50 or so and get an eVGA 6800GS, the 8 pipeline video cards will crunch under pressure from the newer games.
