Re: AGP gaming card
You won't see the graphics acceleration while seeing a boost even with a 128mb model card on the old system there simply by having more memory onboard over some old 32mb 2x/4x model. The latest Radeon model would be too much of an over kill right away!
Eventually when you go for an upgrade at some point as you already know would see even more likely a PCI-E 2.0 card at this time. Besides the card simply adding another 512mb of memory will help a lot there according to what the board supports. Any good 256mb to 512mb video card will suffice while boosting memory capacity will help even more.