Re: what would you guys do?
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.
|Desktop PC| |Macbook Air 13" (Mid 2013)|
|Windows 8.1 Pro (x64)| |AMD FX 4100 Black Edition 3.6 GHz @ 4.0 GHz OC| |Arctic Cooling Freezer 13 PRO| |Gigabyte Nvidia GeForce GTX 560 Super Overclock 1GB| |ASUS M5A78L-M/USB3 Motherboard| |Samsung 840 Pro 256GB SSD SATA III HDD| |Kingston HyperX 8GB PC3-14900 DDR3-1866| |OCZ ModXStream Pro 700W Modular PSU| |NZXT Vulcan Micro ATX Case|