Put the RAM in 1 computer, then buy an AMD card with the money you have, with just one card you won't have any problems with power supply.
Go for scrypt mining(litecoin, dogecoin, etc), bitcoin mining will not be profitable with just one card.
check this website:
https://litecoin.info/Mining_hardware_comparisonAMD is the easiest to mine with and very compatible
Make a decision what you can buy and install 1 card.
-install drivers
-install AMD app sdk (2.9 for 64 bit, 2.8 for 32bit)
-install CGminer 3.7.2(last version to support scrypt)
-make a bat file with pool and settings
Later you can add cards with PCi risers if you have slots available in your motherboard.
If your power supply is not sufficient buy a bigger one or buy a small one and then daisy chain it.
You need special adapters to work with 2 PSU's. or you can use a paperclip and connect green to black (4-6) so the powerswitch is shorted.