Firstly rechargeable batteries are 1.2 volts per cell, so to get decent power (6 volts) you need five of them. Unlike alkalines which are 1.5 volts each, so you only need 4.
The 2700MaH (milli-amps per hour) is the discharge rate and determines the capacity, or "run-time" of them.
Alkalines will start strong and taper off till they are flat, where as rechargeables will hold a strong charge then taper off quickly.
IMO go for a 5 cell NiMh rechargeable hump pack (reciever pack) of at least 1200 MaH, there are bigger ones too (I think up to about 2700MaH ([link=http://www.onlybatterypacks.com]www.onlybatterypacks.com[/link])) which will provide superior run time but take longer to charge.