The 5770 OC comes with higher speeds at stock settings. This can improve graphics performance, depending on the rest of the system. A regular 5770 can usually be overclocked by the user to the level of a factory overclocked model with little difficulty.
If you buy a card that comes with an OC on it already then you will generally pay extra for it as it is a better/faster product than the stock non OC ed card, pretty obvious i know. The thing is this OC is guaranteed and its still possible that you could OC it a little more. These cards usually have better cooling solutions as well which help with the OC.
If you buy a non OC'ed card then you should be prepared to use it as is, overclocking to some degree is usually possible but its not a given. Its different for every card, i have seen cards fall over when given very small overclocks and others refuse to take an OC at all. it happens less and less these days but it does happen.
So your option is pay more and know it will run at the OC'ed settings or take a bit of a punt and do it yourself.
My last 4 cards went like this. The card I'm using now is standard and OC's like a demon, the one before was standard and wouldn't take much of an OC, before that was a His IceQ Turbo pre OC'ed card and was well worth the extra (it overclocked more as well), and finally there was a stock card that OC,ed well enough.