What's your current graphics card?
That motherboard prozac posted would solve your solutions. However, chances are that if you still have an AGP mobo, then you also have a Socket A/462 processor, which would not be compatible with that board.
AGP graphics cards are still pretty good. The 7800GS just came out, and if you look closely enough, you can still find some X850 variants in AGP.
I would say wait until the end of this year to upgrade to a whole new computer. By then, we'll have Socket AM2, DirectX 10, and Windows Vista. Unless you're a hardcore gamer who likes to play F.E.A.R. in HD at the highest possible resolutions, there's really no point in upgrading now only to upgrade again (and spend thousands of dollars) at the end of the year.