Upping the voltage will increase temperature and if you do it wrong it might damage your graphics card. I'd say check game performance stock and overclocked without modifying voltage. Why'd you even want more performance with your uber 5850
The card is uber and if it can be even more uber by overclocking it why not! Right now i basically got it running at 5870 speeds with not voltage increase. Games run great, so guess there no reason to push it anymore if all my games are running fine.