Hi!
1) 150MH/s isn't ridiculously low, there are likely to be plenty of people around who throw less than that at it and are quite happy doing so.
2) No, you probably can't handle those cards with that power supply. Probably 850W+, but the brand quality is also very important. Read reviews.
3) If fortunate, you can get around 500MH/s with 2 5830's:
https://en.bitcoin.it/wiki/Mining_hardware_comparison, if you aren't afraid of overclocking. They're also about half the price of 5850's ($230 vs $110, from a glance on NewEgg; I'm not from the US - your prices seem high). But with a decent enough powersupply and fortunate chips, you could get ~700MH/s if you did spring for 5850's.
4) From what I understand, crossfire doesn't matter for mining - and is better without.
Really, it comes down to how much money you are willing to throw at this, given the ever-increasing difficulty. If you just want to get started and have a tinker with lower risk, buy one card, and skip the power supply. It is unlikely that you'll see any serious return on any investment at this stage - so just keep that balance in mind.
Or you could always just get one card, and stick with your 5770 as well...