Is it possible in cg miner or guiminer to only mine with four of the five Gpus I will have connected?
Yes.
As for the builds you can make it cheaper by running some live usb linux flavor (e.g. Debian) as the OS, so no need for the drive and win7.
You can also save by using a dual core and a cheaper mobo (different socket), if you're gaming for only 4h a month do you really need a good cpu?
No need to buy a case too, more money saved.
I've never used Linux but maybe I'll try it. Maybe I will keep my 2500k, and get a gigabyte z68x ud5, I could add another 6950 with the money...