Newer, larger NiMH batteries (4500mAh and higher) can also offer higher amperage draw and better voltage under power.
check out enerG 4600, Elite 5000 and other larger batteries on the market. www.promatchracing.com
has the enerG cells - they can build you a nice practice pack of unmatched cells which I find to be as good but lower-cost as their matched packs. The Elite 5000s can be gotten from www.cheapbatterypacks.com
Don't skimp on 3000-3500mAh older packs - the newer stuff is pretty good. Both packs (6-cell) would give you prices under 40. Definitely buy two so you can switch them out for each alternate race. On a race day, run heat #1, #3 on one battery and practice, heat #2 and main on the other battery.