Instead of the HD4850 have you considered the newer HD5750 or even HD5770? The latter stretches your budget a bit, but they are newer than the HD4850, they support DX11 and they run much cooler due to their new cores and optimisations. Even the HD5750 should be a bit quicker if not as quick as the older HD4850 if you overclock it a bit.
Regarding the 6-pin supply, todays cards usually include a molex->6 pin adapter with the card package so don't worry about it. Even if they do not give you one, you can buy one for a couple of dollars.
Forgot to mention that if you can't wait, go for the card and enjoy your rig (your PSU should live long enough until you get the money for a proper one). If on the other hand you are after complete stability, want to overclock etc etc, then you should wait until you get a good quality PSU.