aiyah, go to cheapbatterypacks pick up one of the 1100mah 3S lipo for less than USD16.00 lah. Will last a race day for sure.
http://www.cheapbatterypacks.com/mai...owlipos&sale=Y
One thing though, u need to do a little soldering to hook up a plug but the price is right.
magbarnis are right, one of the major reasons to go lipo is that they are relatively light. The weight difference is very noticable if u compare a 1100mah lipo pack vs. a Nimh pack. This makes a difference in long mains (imo).
Regards,
Joe Ling