card: 5850 at 900 clock can get 330 Mh
PSU: just buy a PSU for each card the crap one PSU cost under 10€ and usually are 250 watts
Nice way to ruin your mining operations fast.
Never skimp out on the PSU and buy "crap ones for €10 each". It's the most critical component in the entire rig.
In fact, it's the only part that you should spend extra
money on, to assure you get the best quality one (that will also hopefully accomodate multiple GPU's).
To save money try to find deals on gpu's and motherboards instead. CPUs and RAM sticks can also be second hand, as well as casing.