While you're upgrading it, put it to 512MB.
I can't believe they sold you a 128MB machine only two years ago!! We have an older computer at home (now almost 5 years old), that we got with 512MB RAM.
http://www.crucial.com/systemscanner/
Use that to scan your system, it'll tell you what it can upgrade it to. You can buy the memory from them (theirs is about the best memory you can buy), and you know it'll work with your machine. If you get it from elsewhere, make sure you don't pay over the odds for it. And if you know someone who knows what they're doing, get them to fit it for you. It's not hard (a 5 minute job), but better if you know what you're doing.