I can tell you from experience that not only did my sandra memory benches go from around 390/415 to 480/540 from simply adding 256megs to my asus a7v, but everything performs MUCH faster. Apps open faster. Switching from window to window is MUCH faster. Audio encoding/decoding is MUCH smoother. Video (read: divx) is light-years easier on the system resources usage (like 15% easier on my duron 600@1050).
I was simply amazed at what the extra 256megs did. However, to test, i took out the old 128 megs and just used the Crucial 256meg 222 pc133 stick I had gotten. That was pretty much sufficient for everything I wanted to do, including winrar (700meg .bin+.cue files), nero 5.0.3.8, photoshop 6 all running at the same time. Before, with 128 megs, I couldn't multitask with any seriousness.
Also, from a gaming perspective, Rune, q3, NOLF, Clive Barker's Undying, and Diablo2 all run A LOT smoother. Much more smoothly, like NOTICEABLY!
Anyway, that's my two cents. I think 256megs of ram, at todays price of $89.24 delivered from Crucial is absolutely UNBEATABLE for a system upgrade to your win2k machine. Not only is it unbeatable, it's absolutely ESSENTIAL to getting the most out of your OS.