I have recently upgraded my 8310 to OS 4.5.0.52 and BIS account works perfectly with HTML emails. I am interested to have HTML on my BES account as well, and from what I have read it is possible but it demands some actions from BES administrator etc. Since I am woriking in a bank it is not an option to make our administrator to make the steps involved just for myself. I did alone the OS upgrade.

What is is the best HTML viewer to download with the new OS?

There are threads that cover the HTML viewers out there. The "best" will be whichever one fits your needs best. Read up, try them out and decide.