I use to ask this question too... In my opinion, I feel as long as you have 76 keys, your good to go. There are a few pianists out there who WON'T play on anything but 88 keys. All I need is 76.
On the other hand, since most 88-key boards are usually weighted, as my man said earlier, it is real good to use. A lot of musicians out there now are 'hurting' now b/c they've played so long on non-weighted keyboards. They're starting to get Carpal Tunnel Syndrome. So I'd say, if your 76-key synth is semi-weighted or weighted, ur good to go... if not, u might want to do some more saving and get that weighted action brah...
gea...