your agp 4x slot means you can buy agp cards, which are still pretty good. However, since it is 4x it will not peform to the cards maximum capability. I'm not sure but I do not think that a agp 8x card will work in your motherboard, and if it does it would peform at 4x, so it would be a waste of money really. your best bet is to go with something like a agp card thats 4x/8x, so you can use it in ur computer now, and have it peform at the higher speed if u ever get a newer motherboard with an agp 8x slot.
I reccomend an agp 6800 4x/8x card, seen here:
or if you want a top of the line card (remember you have a 4x slot so it wouldnt peform at its best, but when you swithced to a newer mobo it will peform rather good, and last a fairly long time, and be able to play the latest games for quite some while) This is basically the best card you can get for your system right now from ati:
this is nvidias counterpart of the card above this one. if you like nvidia you might want to go with this 6800gt. im not sure which one is better to be honest, but there very close:
Heres a lower priced card for your system:
and heres a link with lots of cards thatll work in ur system: