To Upgrade, or Not to Upgrade?
OK, get ready for a long answer. :)
Running your Radeon at half speed on the AGP bus won't matter at all for most current/older games, like Max Payne 2, MOHAA, etc. However, games like Unreal2, Savage and (maybe, if it ever comes out ;)), Half-Life2, you'll see a pretty noticeable performance hit. The AGP bus is where the huge memory transfers between frame buffer and video RAM takes place. On super-high polygon and pixel-shader complex games, that memory bus is very important. And, since your buffer is in system RAM, that PC133 will choke on a game like HL2. This not to mention the CPU. Even with the best video card in the world, with a relatively weak CPU like an 'ol T-Bird (no offense), high-polygon and advanced physics engines just won't run like you want. Since you'd actually basically just be building a new system around an old Chassis, PSU and optical/magnetic drives, I don't recommend an upgrade for you.
As someone who builds and repairs PCs for a living, I highly recommend simply leaving your current system in working order, and building a new system around that new video card. You can always give your old system to a relative, make it a server, or sell it to help cover the cost of the new system. In any case, it'll never perform as well as it could with that $400 video card. With a newer system, you could have an Athlon XP with a Barton core (or a 64-bit Athlon, if you're loaded ;)), 400MHz DDR, 8X AGP MoBo with NForce chipset, SATA HDDs, and CDRWs and good DVDROMs only cost around $30 in this day and age. I think you'd be a LOT better off that way, and you'd have an extra system as a bonus. Who couldn't use an extra system? :D