You only need 2 slots, just buy the right amount of RAM to start with.
Remember if you get 4Gb or more of memory, you also need a 64 bit version of Windows to use it properly.
If you're getting a card, see if you can afford a motherboard without inbuilt graphics, preferably a P45.
If you put those boards head to head with the same RAM/CPU/GPU then the results will be almost identical. It's really the CPU and GPU that have an effect on gaming. Both use the same chipset.
Out of those two options I'd go with Asus myself. Higher quality.
I've owned Asus, Gigabyte, and Asrock.
4Gb of RAM will do you fine. I'm still running 2Gb of DDR2-1066. By the time you need more than 4Gb of RAM for games, you'll be replacing the board/CPU anywho.