6990 and 6850 problem

[Maddc]

I have my 6990 mining at about 350mh/s x2 just fine.  I also have a 6850 in the same rig.  The problem is I can mine Either/OR but not both.

I have a 1000 watt power supply, windows 7, ssd hdd but only 512mb of RAM. 

I tried to dump guiminer and use command line from phoenix and again i'm able to get the 6990 or 6850 but not both.  Every time I try to fire up the 3rd card guiminer throws an error.  The miners that are running continue to run just fine, but the 3rd will not run.  The cmd line phoenix miner spit out an error about being low memory.  Does a miner need more than 512mb ?

I put a dummy plug in the 3rd card, assuming that would help, but the problem isnt that the 3rd video wont mine, they just wont all mine at the same time.

Thoughts?