SLI with OC and non OC card

[TurnX]

I currently have a MSI 8800 GTS 640mb card. I want to grab another one but I am only able to find the OCed version of the same card. If I were to set these up in SLI would I have any problems?

Also, another quick question. I am currently using an Antec True Power 550w power supply. Would it be a good idea to upgrade the power supply if I do go the SLI route?