For the rig to be stable, now I use one ps (300W) for one gpu. Cpu/mb/hd use a standalone ps. Before that I have melted 2 ps.
BTW: 4 pieces of 2nd hand 300W ps are cheaper than one 1200W ps.
Unless your 300W PSUs are 80+ gold (do they exist?), the 1200W 80+ gold PSU is going to save you a considerable amount of money over time, and there's no short-game in bitcoin mining if you expect any kind of ROI.
That said, an unstable rig isn't doing you any good, so if it's really the only way you can keep it running, so be it. I have to say there are plenty of miners running multiple cards on larger single PSUs without stability problems, so perhaps this is just bad luck?
The first one is totaly bad luck. The cause of the second one is the HD4850 - at that time I am still a rookie and got no idea about how much power a HD4850 would take...