Well if you don't want to spend that much money but still want to be able to play most games go for the HD4670. Some of my friends have it and it's alright for most games except for heavy duty games like crysis.
What is the budget and do you intend to play more demanding games?
For now I'll say this: The system will easily run a fast card up to the HD4890 or GTX275 but the low resolution of the monitor makes such a card unnecessary: Look for the HD4670 or the slightly faster 9600GT if you want a little more punch. If the budget is larger or you intend to play heavily look to the HD4830/HD4770 from ATI or the 9800GTX/GTX+/GTS250 from Nvidia. Do not bother with a 1Gb card because at the screen resolution even 512Mb is more than enough. EVGA, XFX and BFG have the best warranties and customer support. Read the Newegg reviews, even if you do not live in the US, they are a goldmine of real peoples experiences with the card you are thinking about.