Can I leave in the 128MB ram that is already in my computer, or should I take it out when I install my new 512MB?

You can leave it in and it should work just fine together. Memory doesn't always have to be the same size for each slot. As long as the type matches up, they should be able to work and sync-up the speed (e.g. a PC133 module and a PC100 module in the same machine would make them both run at PC100 since it's slower for one of them, but makes them all run on the same level and be more compatible.)