Not trying to go against the more experienced guys advice here... But... 8 buttons are worthwhile ( necessary even ) for PC games like Street Fighter 4 and its variants, the upcoming SF5, and modern MK games even though menu navigation sucks. If those are games you want to play on your cabinet go for 8 per side, if not 6 will cover the arcade games.
If I were building a cabinet with a widescreen LCD, PC based and that much spare room on the CP I would be looking to play SF5 and Ultra SF4 on it. Its a modern design, so make it modern. Just my 2 cents...
Here's a penny back (though in Canada, we don't have pennies anymore).
You don't
need 8 buttons for SF4, or SF5. Buttons 7 and 8 are only used for all 3 Punches and all 3 Kicks, as shortcut buttons. Most people who are serious about playing SF4, plus people who have played SF from BITD tend to play the regular way without having to rely on those buttons. It is
not necessary to use those buttons for those games. I would say if you're looking to get into Rising Thunder, Seth Killian's Robot "fighting game for the lame", then you absolutely need those extra 2 for Super and Throw attacks.
For me, the only game I would use as an argument for button 7 and 8 would be for Einhander on PS1. I can't think of any other game that would require those buttons AND still work well on arcade controls.