I think it's the Task Manager that allows you to see how much memory is currently being used and how much is available, although I'm not certain as it's been a while since I used Windows. Depending on how many applications you have open at once, you could have your bare minimum open (without GIMP) and see how much memory is available. That will give you an idea of how much you can give GIMP.

To change that you go Edit > Preferences then in the Environment bit there's a bit where you can set resources consumption. I probably wouldn't set it more than 75% of available RAM from above, unless that would make it less than it currently uses. So if it was 2GB available then I'd look at setting it to 1.5GB tops. It's useful to keep some memory free for system tasks and that. At least that's what I believe anyway. I could well be talking nonsense though as my perception of these things could be well out of date. I just know that Windows tended to be quite resource hungry when it came to background housekeeping tasks. Oh and it had a habit of using the swap file regardless of how much free RAM there was.