I've only installed 7 as a clean build, so can't help with Q1.
Regarding speed, what exactly are you hoping will speed up in MAME? the games should run as close to the original game speed that you hardware will allow. If you have games that run below original speed you need to identify what is slowing it down and try to erradicate it. Usually the limiting factor is is your CPU, and from my experience, 7 doesn't magically free up some spare CPU cycles, although as an OS is is definitely nicer to use on a daily basis than Vista (maybe XP too...?).
Thats one of the reasons my MAME machine runs Ubuntu Linux with SDLMAME and WAH!CADE rather than windows. I built it to do a limited set of tasks. These being: a file server for my HUGE movie collection (it feeds all the TVs in my house), a Jukebox, a torrent downloader, an arcade machine and a DVD ripper.
It does all of those perfectly. Not a single cpu cycle or byte of memory is wasted on antivirus software (before any windows users jump on me for not running antivirus software, read
THIS ), anti malware, desktop widgets, automated online backup software etc etc and all that gubbins that windows seems to accumulate over time, just doing nothing.