Is there a difference if i oc my ati 4650 in crossfire???

Depends on how OC-able the cards are but yes, you can see very big performance increases from OCing. Just FYI there's often an issue when OCing in CF if, for example, you use MSI Afterburner with the option checked to copy the settings to similar cards... You pretty much have to manually adjust each cards clocks separately. It can cause freezing or BSOD if you try both at the same time. (So, overclock 1, select other card, and set it to the same speeds). Easy to check using FurMark or gaming benchmarks that you can download for free.
 

Pyroflea

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You can do either. If you want to save power/heat, and reduce noise, then I'd just run it at stock speeds when you're not using it. I use Evga Precision (for Nvidia cards, but just using it as an example) to OC my card, and it allows to you set profiles. So I have stock speed, and then overclocked. I just apply whichever I want to use :)