Well, state of the art is PCI-express (which your MOBO doesn't support), and the fastest cards are around the £250 mark.
Previous generation Radeon or GeForce are AGP 8X and will set you back about £100 +/- a tenner or so (new). This will get you a card with at least 128 MB VRAM, DVI and TV-out as well as std. VGA.
Is there any point in getting an earlier generation AGP 4X? Depends on the price - you might pick up something better than your current card for £30 - £50, but it will become (more) obsolete quicker!
SD