I voted for the 770, because I already own an N800 and really miss the 770. It was prettier, hat a better form factor and, above all, was the only truly mobily device in the whole series because of its hard cover. I keep looking for a cheap 770 in my area as a second tablet for travelling.

For you as a new user, however, I'd recommend a N800/N810. Believe me, you will want the additional CPU power. You will be happier with OS2008-based software. (Although UI-wise, the 770's OS2005/OS2006 were better.)

As for hacks:
Yes, there were hacks and neat applications in the early 770-days that never got ported to the N800/N810. The way Nokia handled the transition from 770 to N800 caused a lot of frustration among developers, so some left the boat. But that doesn't mean there's less for the N8x0, the opposite is true. A new generation came along, and what you can do now with a N8x0 is just amazing. (Don't forget: The N8x0 is 2 years old now, the 770 was replaced after only one year and a half.)