If you get a new computer, it will probably have Vista on it, which (take my word for it) you DO NOT WANT. It's very sluggish and takes a substantial amount of tweaking to make it usable. I had Windows XP on my old computer and liked it a lot better. I'd say your best bet is to just buy some more RAM. You should be able to get another two gigabytes for about $30 at Wal*Mart, Best Buy, Circuit City, or lots of other places. It's also easy to install. Just make sure that you get the kind that will fit your computer. Laptops and desktops use different sizes.
Edit: Plus, if/when you do get a new computer some time in the future, you can take the RAM out of your current one and put it in your new one.