You've got a bit of a problem there. That sort of PCs usually come with small PSUs and small cases, both of which could prevent you from upgrading to a serious gaming card. Do you have the PC already? Try to see how much room you have for the video card. You need about 9.5" for a HD 4870 and about 11" for a GTX 260, I think. Also check the PSU parameters - they should be written on the back of the PSU. You need something like 500W or more for HD 4870, and 525W/(38A on 12V) for GTX 260.