Hello,
More RAM is better than less RAM. Assuming you have only 2 SODIMM slots on your laptop's motherboard, then with or without the extra 512MB RAM module installed, dual channel won't be applicable because there aren't pairs of the same spec RAM modules installed.
It is fine to install different capacity RAM modules in a system. Differences in capacity of RAM modules won't impact performance (with exeption of a dual channel configuration which will give you a slight performance improvement).
However, the clock speed of the RAM modules will have an effect on performance (eg DDR2-533 / PC2-4200 etc). Ideally you'll want all RAM modules to be clocked at the maximum speed the motherboard can handle. So for example, for a laptop motherboard supporting maximum RAM speeds of DDR2-533 / PC2-4200, you'll want all memory modules to be DDR2-533, to benefit from the fastest speed there is to offer.
