if you ahve that kind of money get a motherboard without AGP slot and buy a PCI Express graphics card as it's much better.
If you go with that board you will probably need to upgrade your RAM and CPU too.
Make sure whatever board you get has enough regular PCI slots to support the cards in your machine that you have now and look for one with on board sound and network card if you don't have cards for those already.
i could intimidate you with specs all day about what to get but remember that buying a brand new board will mean a new CPU and RAM also for you if you PC is that old it doesn't have an AGP slot.
800 is enough to get a new board, gfx card, ram and processor and maybe even a nice case.