The motherboard comes with a AGP 4x slot. I'm looking for a card with 256M that supports Shader technology. I wonder if Nvidia 6600GT is a good choice... coz I'm not sure if the motherboard will allow the card to run freely or it'll be a waste...
Well, tests have shown that pretty much any AGP card will still work fine even for AGP 4x boards, and most certainly won't be "wasted."
As for the card to get... Unfortunately, ATi's rediculously-priced-direct X800XL and X850pros are gone; those were, by far, the best deals ever to come for AGP.
The GeForce 7600GS is a potential good choice, however, some people are confusing it with the 7600GT, when there's a MASSIVE performance difference between the two; the 7600GS is more comparable, it seems, to something like the Radeon X800GTO or the old Geforce 6800GS; cards such as the X850pro or X800XL would provide markedly higher performance, in the 25-30% neighborhood. However, they may not be available at a reasonably comparable price.
You mention "shader technology," which is perhaps a little... Vague. all current cards either support shader model 2.0b (the Radeon X series) or shader model 3.0. (Radeon X1k, Geforce 6, GeForce 7) Both are well beyond the SM 1.3 of the GeForce 4 Ti, and even noticably above the SM 2.0 of the Radeon 9 and GeForce FX 5 series.
The main difference between the two models (2.0b and 3.0) are in what forms of HDR (high-dynamic range lighting) that they support. SM 3.0 cards support all forms, while SM 2.0 kinds of HDR are only used, or are availible for, some of the games that use HDR; Half-Life 2's HDR is SM 2.0, Oblivion has a mod to enable SM 2.0 HDR, and, as I understand, Splinter Cell 3 has a SM 2.0 HDR option.