I read somewhere that w2k and wXP has its optimal ammount of memory at around 1024mb. =1gb
Complete load of cobblers.
Win2k and XP, a decent configuration (for home needs anyway) including drivers and all patches should weigh in between 30 and 50MB RAM used, depending largely on drivers.
Anything after that depends on your use of the computer, take a look at Task Manager's total memory used. For average app usage, I'd recommend 128 - 256MB. For games, up to 512MB. Beyond that you should have a specific need for the additional RAM.
If you do have loads more RAM than you need in a WinNTx (NTx = NT4, Win2k, XP) there are a few tweaks you can do to increase performance, which are not done automatically. Can't say I've noticed a big performance difference using the tweaks, but in theory they should make some difference.