Your first problem is getting a card to fit in the slim case that will max game settings.
The second problem is the power supply unit, which is nearly always a proprietary PSU. It would be nearly impossible to replace a proprietary PSU with one that would power a high-end graphics card.
Well heheuehueheueheuheuh I ther a pc together it has 2 248mb ddr3, ati radeon 9600XT 128mb, amd athlon X64 x2 at 2.00ghz, an asus KV8 deluxe mobo, the cpu is gonna be oc'd at 2.8ghz when I get an hdd with windows 7 on it