Just to clarify something on the first page, if the new RAM is capable of higher speeds than the old one, then the new one will run at the maximum speed that the old one can, a bit like Gunner's illustration.
Crucial may say that it's alright to mix and match RAM how you like, but that may be using just their brand. There are quite a few instances out there where mismatched RAM has been a problem.
As long as the two sticks are compatible with each other, then your computer will definitely recognise all 1.5GB. You will know if they aren't compatible, because the PC will not boot. If this happens then take the 512MB stick out, put the 1GB stick in its place, and try again. Then personally I would put the 512 stick on eBay to get some money towards another GB
