I don't think that ULi chipset has AGP lock. Here's an idea for you... buy a Fry's/Outpost Sempron/ECS combo. One of two boards available is an ECS NFORCE3-A that many people have had great success with clocking to the BIOS maximum of 250MHz HTT/FSB. That would put your CPU at the exact speed it used to run at. The combo has been as cheap as $70 with a Sempron 2600/2800+, and often around $80 with an OEM Sempron 3100+. What you can do is get one of these cheap combos and sell the Sempron CPU. I'm sure that you can get at least $30-40 for the CPU which would offset quite a bit of the cost of the motherboard.
Such a combo would give you exactly what you are looking for, ability to use all your parts at what they are capable of running at, while being as cheap as possible to save you money... that you'll end up wasting on Vista. 😀
Heck, if you got such a combo and it came with a Palermo chip, I'd be first in line with a $30 offer for it. 😉