I'm pretty sure any AGP card you get (as long as your AGP port supports the appropriate voltage) will be backwards compatible with your 1x slot, it'll just lower the card to that amount of bandwidth. In other words, if you bought a 9800 Pro, it would run at 1x, not 8x, and you would probably take a hit in performance, though I'm not sure how much of a hit.
Keep in mind though, judging by your motherboard, the fastest CPU you could possibly have right now is 600MHz. That's going to be a huge bottlekneck on anything much faster than a TNT2 that you put into your system, so you likely wont see leaps and bounds in performance inprovements.