Which graphic card: ASUS EAH3870 TOP Or ASUS 9600GT

Hello,

I am building a new system, but i don't know which graphic card i must choose.

This is my system so far:

Intel core 2 quad Q6600 2.4 ghz
Asus P5KC mobo
Samsun spinpoint 502IJ 500 GB
Take MS 4 GB DDR2 6400 800MHZ CL5

Is this a good system? and which graphic card fits well in this system?

ASUS EAH3870 TOP/G/HTDI/512M the EN9600GT/HTDI/512M or the EN9600GT/HTDI/1G ??

Already thanks!!!..

Greets. Fito