upgrade your psu to a 80+ rated, active pfc one with 30-40~ A on it's +12v rail and get the gigabyte 6770 (for your current res.) or his iceq 6850 or gigabyte's 6850 (whichever supports cfx).
with your current psu, nothing more than a 6670 1 gb gddr5.
your 2.4 ghz cpu will bottleneck a cfx setup and may be a 6850. you might need to upgrade your cpu and motherboard in the future to take advantage of cfx/sli.
I will sell this computer and buy a new one.I will build and i have already choosen the parts i just need some time and some good prices
So which of this : http://www.amazon.de/HIS-Radeon-6670-Fan-Grafikkarte/dp/B005FYH24S/ref=sr_1_8?ie=UTF8&qid=1325082357&sr=8-8]HIS
Radeon HD 6670 Fan - graphics adapter - Radeon HD 6670, H667FS1Ghttp://www.amazon.de/GigaByte-Radeon-HD6670-Grafikkarte-Speicher/dp/B004ZHDZUG/ref=sr_1_9?ie=UTF8&qid=1325082357&sr=8-9]Gigabyte
Radeon HD6670 OC video card (PCI-e, 1GB DDR3, DVI, HDMI, VGA)
OR i should get one form nvidia like http://www.amazon.de/Asus-ENGT440-1GD5-Grafikkarte-Speicher/dp/B004L3WVI6/ref=sr_1_5?ie=UTF8&qid=1325082617&sr=8-5]Asus
ENGT440 DI 1GD5 graphics card (PCI-e 1GB DDR5 memory, DVI, HDMI)http://www.gpureview.com/show_cards.php?card1=654&card2=643
(here is a comparission of them)