The 820 will overclock "fine" but it wont' be anywhere near a C2D in performance. Even an 80% overclock on an 820, for example, will get spanked by a 20% OC'd 4300. Just keep it in mind. So it'll overclock "fine" but its performance overall will pale in comparison to the Core2Duo chips.
Here's how to OC on the 680i......more geared to the C2D chips, but you'll get the idea though: