Almost any card you buy is going to come with the Molex converters. As for power supply, you _might_ be able to get away with the 750, but I doubt it. A 5830 is going to pull 160-175W at full load, so figure up to 700W for just the cards.
I'd opt for a 950 or so just to be safe, but you do have a few other options.
First, pick up a Kill-a-watt meter if you don't already have one. Start up your rig and see what it's pulling at full power, then stop one of the cards and see what you drop down too. Might give you an idea if you're close.
Other option is to just pick up a second PSU, something small like a 450-500 and run one or two cards with just that PSU. It's pretty easy to short the PSU pins out to have the second PSU power other cards without being hooked up to the motherboard.
I'd suggest trying it with 750, but expect you'll need a bigger or secondary PSU.