According to Crucial's memory advisor, your laptop should have 2 slots (one containing the 512MB module & one empty). The laptop can support upto 2GB in total (1GB per slot).
You should be able to simply install a 1GB module to give you a total of 1.5GB, however you won't be able to take advantage of dual channel mode as the 2 modules need to be matching for that. Also, the closer the 2 modules are in terms of specs, the more chance they have of being fully compatible.
If you want my advise, it's this - buy the 1GB stick from Crucial. To start with, Crucial guarantee that the stick will be 100% compatible with your system & secondly, they offer a lifetime warranty on all the modules. Crucial may be a bit more expensive but it's never worth using cheap memory just to try & save a few bucks.