> 1. Given that I have a 64 bit processor should I use a 64 bit OS & mame build? What
> sort of performance gain should I expect?
Yes. The gain is between 5 and 20% in MAME, which can be significant on heavier-weight games (e.g. newer Donkey Kong with the proper analog sound effects simulated, or 3D games). Some future optimizations the devs have in mind may be 64-bit only or most effective on 64-bit as well.
> 2. Given that I have plenty of HD space (250gb). I currently only have 1gb of ram but
> not a problem to install more should I even bother with a Tiny/Mini version etc?
> Which OS generally gives the best performance? XP or Win7- I guess are the two front
Win 7 x64 runs MAME extremely well, being it was made by Aaron Giles ;-)