Well, I'd personally go with an X1900.. either XT or GT. They are very similar.
X1900GT = 12 pixel pipelines, 36 pixel shaders
X1900XT = 16 pixel pipelines, 48 pixel shaders
The 7900 series is 24/24.
Pixel shaders are for executing shader code. BF2 makes light use of shaders, Source Engine makes light to medium use of shaders. Newer games like Oblivion make massive use of shaders, as do the number of new pixel-shader rich games on the horizon.
Pixel pipelines are for rendering pixels/normal textures and accomodate fill-rate. Shaders execute first so a shader rich 3d engine may not be able to achieve it's full fillrate potential if there is a bottleneck in the shader pipelines. This is where ATI has focused their products- assuming more and more complex shaders will replace fat, beefy bitmap/textures for game assets.
I'd probably lean more towards the GT mainly to save cash. Realize whatever you buy today may be obsolete in 2-6 months as the new DirectX 10 hardware hits the market.