I found your PC but what I didn't find is the PSU. I need that to recomend a nice GPU for you. But your PC has 2 cores so you will be limited to how big of a GPU you can go.
Most store bought rigs come with a small PSU. On avg it is a 300w generic PSU. If it is that I beleive you can get into a 6670 if the amps on the 12v rail is high enough. Which in most cases they are.
Now if you up grade the PSU then I believe the biggest card on AMD side would be the 6770 and a bif iffy 6850 depending on how big or core duo is. Now on the nVidia side would be the GTS 450.
And maybe a iffy GTX550 Ti. I saw the other day a two core CPU run well with the GTX 550ti. So to be safe I would go with the GTS 450 or the AMD 6770 and up grade your PSU to either a 430w to 500w PSU.
There is one thing I forgot to ask what is your budget? All the stuff I listed is priced. I hope this helps in some way and Good luck to you