Oosel:
The 7 cell 2400 MaH Nicads will work fine, and because the Hacker is more efficient and more friction free, you will get longer flight times than with your brushed motor. I forgot what you are using, but you could expect as much as 50% longer flight times with the Hacker ... or maybe any brushless Motor. Naturally, packs with higher MaHours will give you longer flight duration, but what you have will work just fine. I've heard of guys that use 1700 MaH packs!
I'm getting 10 minutes or a tad more with 2600 MaH NiMh packs.
Happy Choptering,
Russ