I seriously dont get it ..what is best ?...a ps3 graphics card ...(which i heard was like 8800gt) or the xbox 360 custom ati card which is equivelent to the (4870) i think ...........or custom built pc i have both xbox360 a and pc with crossfire sli (ati 4870 ) but i dont get it why does the xbox look so good and how come it runs smooth ...does that mean a custom built pc is better? O_O

A custom built gaming pc is much better.
A good CPU is the Intel Core 2 Duo.
I have a 9800GTX+, which is excellent for DX10, but depending on your price range, a single GTX 200-series or 400 series may suit you better. The 9800GTX+ costs 99 USD, so an SLI is about 200 USD. However, you might have to get a new motherboard.
If you are willing to spend $280, you can get a GTX 465, which is excellent.

A 9800 GTX+ SLI would still beat your current setup.

If you are willing to spend a lot of monery, then here are some excellent specs:
CPU-Intel Quad Core i7 930x Factory Overclocked Extreme Edition
GPU-nVidia GTX480 3-Way SLI
Motherboard-Asus Rage II gene
Memory-2 Sets of Patriot G series ‘Sector 5’ Edition
PSU-Cooler Master RSA00-AMBAJ3-US