Well DDR400 only significantly outperforms DDR333 if the CPU is run @ 400mhz bus, synchronous to the RAM. I don't know that much about Tbirds since I skipped from Duron to XP in my last upgrade. But I would say it's probably difficult to get your CPU stable at 400 bus. I have a 1700+ Tbred which is really cheap and factory unlocked by nForce2 so you can underclock the multiplier and increase the bus without actually running the CPU faster than stock. Tbred 1700's are I think $50 shipped at New Egg or something like that. But if you are sticking with your current CPU for a while, I would say go with a 512 stick of XMS2700 since the CPU is going to be the limiting factor and the 2700 RAM is a bit cheaper. But if you're going to upgrade the CPU sometime soon, get the XMS3200 and run that mofo @400 sync!!

That's what I'm doing and there was definitely a noticeable performance jump.