need advice with choosing power supply configuration

[alex_g2k]

i'm a new guy in led-lightning and need some advice from experts here. i'm bilding a cree p2 based headlight for my bicycle. i don't need super-bright set-up, just a bit brighter then original halogen lamp i have installed for now. my current headlight has 4xAA ni-mh with 2.4w 4.2v halogen lamp. i want to replace halogen lamp with cree led. i've bought romisen rc-g2 recently and i'm very pleased with light it provides. this little cute flashlight inspired me with the idea of upgrading my bycicle lamp to cree-led. i want to keep original headlight's body, cause it can hold 4xAA batteries and replace lamp, reflector and optic parts. i ordered 700 mA driver and q5 led for my romisen to make it super-bright (not fo bycicle, but just for fun) and i'm going to use it's original driver and p2 led as installation components for bycicle headlamp. i ordered a reflector with a heatsink and special shaped optics which i expect will spread a light beam horizontaly. as i prefer long runtime to extra brightness, i assume that original romisen driver with it's 500 (or so) mA will be quite enough. as it is designed to have one AA battery as a source of power, the only one way to use that driver with four AA is to connect them parallel. and that's where i'm a bit confused with problem whether to use 1.2v romisen's driver with parallel-connected ni-mh's or order a new one which required input voltage range contains 4*1.2v of ni-mh's and connect them serially. what would you reccomend? which configuration is more efficient?

 

[alex_g2k]

so still need some suggestions about batteries configuration to choose: 4xAA in series or in parallel with appropriate driver of course.
an have got one more question about efficiency of lumen/watt for leds. as far as i can get most of the led's have maximum of lumen/watt not at nominal current but at much less than that. e.g. 20mA leds are more efficient at 7-8mA. am i right? so in case i want to achieve the maximum runtime for an acceptable light intension, i should use more leds with less current flowing through them then their's nominal?
so if instead of plugging one high power led to 350mA driver plug say 30 x 5mm leds in parallel, would i have get more lumens from that?

 

[alex_g2k]

still need help. please answer me if you know the solution of my problem.