Well, you could take the option of
putting a real operating system on it!
I assume you game a lot so Linux isn't an option?
OK, so you could choose to buy the RAM later in the expectation that DDR2 prices will continue to fall or, at worst, stabilise (a slightly risky strategy!), or buy that RAM and either a) leave it all nicely wrapped up until Windows 7 gets released (a very frustrating strategy), or b) see if anyone has a Windows 7 Beta iso that you can borrow, do the upgrade and take the slight risk that something will go horribly wrong while you're using the beta. Obviously, that last one is the riskiest! There are plenty of known and published issues with 7 Beta now though, so a good google trawl should give you plenty of information (just for a starter, I can tell you now that TAGES DRM-protection doesn't work with 7, so if any of your games uses that it's probably not an option).