Pixel Shader 4 is in DirectX 10.
While you won't be able to order from them if you're not in the US or Canada, a good starting place is the power search function at Newegg:
Newegg.com - Computer Hardware, Video Cards & Video Devices, Desktop Graphics / Video Cards, PCI Express x16, DirectX 10, PCI Express 2.0 x16, Direct...
You need a graphics card for a PCI-E X16 slot, supporting DX 10, 10.1, 0r 11.
Additional considerations: your power supply must have enough capacity for the new card. (400W is typical, even for the low end.) If you get a midrange or higher card, you may also need auxilairy PCI-E power connectors.