I wouldnt upgrade just yet, with an Athlon 3000 and that Gfx card ALL games will be playable at the moment and in the near future.
That is an AGP card and as such is virtually at the top of the pile, the 6800GT is probably the best of the last(affordable) AGP series of cards.
All of the GFX card companies now produce there cards in the PCI-E format, so unless you have an PCI-E capable MB what you have got is still very capable.