You'd have to upgrade at least the cpu& memory together. Considering the cost of 1 GB, I'm not sure its worth it without upgrading everything at once. You could plug in a barton 3000+ and get some 400 Mhz DDR, but it would be a rather steep price for a smallish upgrade.
Or you could search for an unlocked barton 2500+, run it a fsb as high (well, low) as your memory will allow (333 if you're lucky), and increase the multiplier as high as you can. it won't bring that much, but the extra cache and clockspeed should help, and its definately not expensive. If you've got low latency memory, it might run at 333 with acceptable timings, and the 2500+ would probably overclock to 2-2.2 GHz which would get you ~3000+ performance for $70 while keeping your gig of ram. that might be worth a shot.
= The views stated herein are my personal views, and not necessarily the views of my wife. =