PHPBB, for a lot of reasons:
Free, highly moddable, lots of themes, advanced, blends into portals, fairly easy to install and a few more but all the thinking is hurting my brain. Basically PHPBB owns.
Nope. A good restart makes your PC faster, and leaving your PC on for long enough slowly kills the CPU. I play a lot of online games and I find myself restarting every 2-3 hours to achieve max. speed.
I used this for a while, I uninstalled it because it didn't make the difference I thought it would. But, it definately did speed up my 10MB broadband slightly.It downloads things your browser is likely to request next, the key phrase. On a large broadband connection the extra background downloading is unlikely to make a difference while it is happening, which means your experience probably will be faster, but a dial up will really be feeling the background downloads and they will potentially slow it.