Ok so I've done a little bit more research, that battery thread really helped. This site has allot of very good info.
So I think I want an 8.4v or 9.6v battery(ies) with allot of Mah. You guys seem to say not to buy them from an Airsoft store, why is that?
Also good M4 brands look to be ICS, G&P, CA, TM, and some people say ARES (but reading earlier post.. crappy internals, but I'll do some more searching)
I like the CA version, seen good reviews.