Win2k is sometimes referred to as NT5.0 & WinXP as NT5.1, that's how similar they are.
No, actually, they are referred to as NT 5 and NT 5.1 because those are their version numbers.
In an interesting twist, the difference between Windows 2000 and windows XP is the same as the difference between Windows Vista and 7, or approximately so; this therefore brings up the question of why many people think that Vista is more useless then a chocolate teapot?
Anyway... I ran windows 2000 with... 64MB, it was livable, but you had to be sparse with your demands.