2nd GPU?

So I have a Radeon 5850 Black edition that i got for $50 and I can get a 2nd one for about $35 dollars and I was wondering if it would be worth it buy the 2nd 5850 and buy a new motherboard to crossfire the cards.  I was also wondering if i would need to buy another PSU if i have a 500 Watt PSU.

Not worth it in the least bit. If you had the conditions met already yes, adding a cheap card might be worth it (if you really can't save some money and be patient for a better card), though it can cost more on power in the long run (and add heat and noise). 500W+ is recommended. You are pushing it way too much, since two cards consume 450~ W at full load.