The ASUS P4PE-X motherboard uses an AGP 4x graphics slot.
Windows 7 is independent of the graphic interface. You just need a graphics card with DirectX 9 or better to run the Aero effects.
This is one of the most powerful and modern AGP based graphics cards you can buy.
HIS HD 4670 1 GB AGP - $99.99 after rebate ($124.99 before rebate)http://www.newegg.com/Product/Product.aspx?Item=N82E16814161284