considering that you are only going to spend around $140 now for improvement of that system then i don't see an issue with that!
but i must say that their isn't a better time to go with a PCIe setup as prices are insanely cheaper and the performance over AGP will just blow you away. it has me since i made the leap early this year!
if you decide to do this upgrade for your AGP setup then it should be your last one and not for more money as it wouldn't be worth it then.
For $300, you can have an ASUS motherboard.
AMD 4200 X2, 2 gig of ddr2 memory, and an 8800GS.
Dig up a couple hundred more bucks, and shop Newegg a little.
I myself would not spend money on anything AGP, when everything else is so cheap to upgrade and give you so much more.
The GDDR2 vs GDDR3 is a MUCH bigger difference than I would have thought. Don't buy a card with GDDR2 Memory. I would go with jitpublisher's recommendation if you can. It will be a huge boost, last longer, and be more upgradeable in the future.
instead of throwing good money at an old dead rig, spend $300 - $400 on new core components instead. new mobo, proc, ram, video card. remember, it seems like an easy upgrade now, but maybe the ram won't be compatible, or maybe you're old mobo decides that it aint gonna work anymore a month down the road, and good luck finding an amd agp mobo to replace it. if you're smart, sell what you got on ebay and upgrade the core components.