Well Ultimate is wated. There is no Extras that make it worth the $$. Home PRemium can do Media Center. Home Basic is worse than XP Home edition. Steer clear of that. So it would be your choice between Business edition and Home Premium. I would say Home Premium.
as for making it faster. There are a bunch of TweakVI applicaitons out there. Try a couple of those.
As for a firewall...that is tough. Comodo isnt compatible yet. Most of the other ones i know of are part of a security suite. Zone Alarm is the ony one i can think of off hand that is just stand alone.