I think that you have been mis-informed! Windows certainly doesn't need 2Gb for best performance. The estimates that I have seen give a figure of around 390 Megs for general usage, suggesting that 512 Mb of RAM is more than enough.
For graphics usage, the situation is quite different. Here 768 Megs is OK.
Only for video rendering applications do you need more, but even then, there is a limit. In this case, throughput is more important - meaning that the busses and such motherboard issues play an equally important role to the RAM.
Linux is more memory intensive simply because it is a virtual memory system! The more memory you have, the more you can fill it up! Even so, 1 Gb should be more than enough for average users.
2 Gb and greater, is found on servers where possibly thousands of simultanious connections are possible. In reality, it is doubtful if such huge amounts are ever used.
Your question would be more sensible, if you told us what you had in mind usagewise. Any answer would be hardware dependent in any case.