I wouldn't. At least, not right now. The geforce6800gt is plenty powerful for the latest games as-is. The 6800 cards can also run a bit hot at standard, so overclocking them is a gamble (factory heatsinks usually max out at the normal temp of the card).
Once we see how HL2 is going to (globally) tax your system, and any future games, then you might want to think about OCing that sucker. I'd definately think about a new heatsink and fan for it, though, when you do. (Watercooling is great for OC'ed g-cards, too, but that's expensive!)
You'll be happy with the card as-is. If your system is up to spec with the card, you'll be fine. Overclocking the card on a slow system is not going to give you a performance boost anyway.