I upgraded my Thinkpad from 768MB to 1024MB. Graphics can use up to 64MB but most of the time (not playing games) it only uses 8MB of RAM. Why did I upgrade? Simple reason: I had the money and wanted to "even" it out.
I bought it at Fry's (a 512MB stick) for like $50. I can't remember the exact price as I think it was in February or somewhere around then.
Did I see a big performance leap? Hell no! Has anything improved since I've upgraded? It seems, IMO, that programs like Photoshop start up a bit faster. Gaming wise, I can't really tell but I don't play demanding games on my Thinkpad.
Was it worth it? Well, you can never have too much memory.
It maxes out at 2GB so I might be doing that soon, but that involves buying two 1GB sticks to replace the already twp 512MB sticks in there.
The biggest improvement I've seen is in the upgrading of my 4200RPM hard drive to a 7200RPM. :drool: