Just how much memory is enough?

Aigghhh.... i think it's time for my next computer memory upgrade. Every program that i use everyday is so memory-intensive that my system is running like a sloth right now. Just how much memory is enough? 1Gb? 2Gb? Or should i max it out at 4Gb once and for all.

And i'm still using Windows Xp. If i switch to Vista, i dont think this thing will even startup without hanging. I heard that it's even more memory intensive. *shudders* Or maybe i should consider ditching this system and get a new one altogether. How to work like that....