I just upgraded my computer's memory by 512 MB for about $100. It's a great decision; I regret not doing it earlier. In fact, I'm likely to upgrade it another 400ish MB (replace the 128 MB card with a 512) in the next year.
I did it through crucial.com, which I highly recommend. Crucial.com is a division of Micron Technology. They make it easy to find the right kind of memory for your computer, which is the hardest part. They offer reasonable prices and free shipping.
Their support was also excellent. Highly recommended.