200$ gaming PCs are hardly "gaming".
By APU you mean integrated graphics? Because those are generally only good for office use. They might be able to handle a few older, undemanding games, maybe, but definitely nothing that's even relatively new. You will get neither smooth gameplay nor graphics.
Micro-ATX has fewer slots for stuff, so make sure you have enough for what you need.
You might get better performance from 2x2 RAM sticks, if the other stats are the same as the 4GB one (due to dual channel usage, not sure about this though), but it would make upgrading in the future more limited or at least more expensive, since you want to stick RAM sticks of roughly the same stats in your PC. With 4GB you can later buy another 4GB, and then some more, if you wish. With 2x2GB, you'd either have to discard them in favor of 4GB or greater sticks or buy more 2GB ones, thus limiting yourself to 8GB (assuming MicroATX has 4 RAM slots, i don't know about this).