Hmmm, I had to use my older system recently with almost the same setup. I tried some games on it like Jedi Knight II. It wasn't playable. At least not at the resolutions I'd become used to... (800x600 was quite choppy)
If you didn't want to upgrade the mobo, the AGP Pro may give you compatibility issues with today's AGP graphics cards (see Wikipedia entry). And the higher end ones cost more than a PCIe Mobo and mid/low end graphics card and memory. Also, graphic cards need to be balanced against the CPU anyway.
But you probably already went through that thought process!
I suggest that you peruse the motherboard review thread at the top of the motherboard forum.
http://forums.anandtech.com/me...=2067695&enterthread=y
Recommendations on High End/Mid Range/Low end mobos with some explanations on the chipsets.
I'm using the recommendations on microATX at the moment as I want a semi portable rig.
Please bear in mind that mid September, there is a new chipset out from Intel for Intel's Core2Duo chips. This will probably drive some mobo prices down. But it's not that expensive right now to get a mid level system.
Anandtech is a little busy with their massive micro ATX mobo roundup so the articles/reviews haven't been coming so fast. So pop over to ars technica and look at their value gaming rig if you want more specifics.
http://arstechnica.com/guides/buyer/guide-200707.ars/1
Hope that this helps.