Most modern graphic cards are PCIe only, and your motherboard has AGP slots, so if you want to upgrade your graphic card, it is almost assured you'll have to change your motherboard too, and that means that you'l have to change your processor.
The games and apps you want don't have high specifications, so you should be fine with a not so awesome computer. That said, all of them have crap-ass graphic cards, which is the most needed part in a game, so I'd suggest buying one of them (the second or the third) and then buying a decent graphic card for it.
Right now, a NVIDIA GT 9800 or a GT 220, or an ATI Radeon 2600 are pretty low cost and should give you enough to run those games without any problem with high quality settings.