A RAM upgrade is definitely something you can do. On some of the newer laptops like the 700m you can even upgrade the processor though it's not supported IIRC.
Find out what type of RAM you need. I think on the 600m it's standard laptop PC2700 (The dell support site will have this information.) Then order the memory through your
favorite online retailer so you don't have to pay a huge markup. There is a caveat though:
Some laptops come with two memory slots, one internal to the chassis and one externally available. You'll want to find out how the RAM you have now is setup. Do you have 2x 256MB sticks (one internal and one external?). Do you have 1x 512MB stick in the external slot? Kknowing this will help you figure out how much RAM you need to order and whether or not you have to replace the stick you've got in there now. In the case of my 700m, for example, I just had to order a 256MB stick and throw it in the external slot to get 512MB total.