Popular Models for both Slots
The most popular models are Nvidia and Radeon, so we will look at these; there are other cards available which are equally as good.
If you still own an AGP-only motherboard, you could consider buying a Radeon 9800XT Pro or an Nvidia 6600GT which are good cards. They won’t run titles which are too new (Assassin’s Creed or Dragon Age Origins for example) but if you are terribly outdated they are good upgrades.
Better cards which will indeed run next-generation titles are the Radeon HD 4650 or the Nvidia 7900GS, both great cards for older systems. However, some users have speculated that it may be problematic running these on an older-socket system even though the requirements for both cards do not specify anything at all (apart from power-supply specifics).
If you have a newer PCI-E 16x slot, it will be easier to shop around for new GPU’s. Depending on what you want, i.e. budget models vs highest-performance, the Nvidia Geforce models are still going strong (i.e. 7800, 8800, 9800) for gaming PC’s. The newly released 100 to 400 series offer incredible visuals and performance. Radeon still provides various solutions, such as the Radeon 3000, 4000 and even 5000 series products using the latest “Evergreen" Graphic Processing Unit.