At resolutions over 1024x768, the CPU matters less and less. It's the graphics card that will bottleneck gaming performance, so don't worry about that Athlon64 slowing you down.
The 7950 GX2 is great, but it's a waste unless you're running 1600x1200 with high levels of AA.
At lower resolutions (like 1280x1024 with 4xAA) you're just as well served with a single 7900 GTX or X1900 XTX.
Your best bang for the buck is the X1900 XT 256mb. Only $240 on newegg, and in the same performance league as the 7900 GTX/X1900 XTX...
...unless you really need the super-high resolutions and AA that the GX2 can provide.