The differences is 2000 and 2400 have more MAH (duh) and don't have to be recharged as often. I'm not too sure about voltage on 2400 cells because I got out of electric racing when they came out.
Anyways, 1700 cells do in fact offer more punch than 2000mah cells. I know this because I would run 1700 cells in stock class while I ran 2000mah in modified class. The way you charge the packs also affect how much voltage they initially put out.
But I wouldn't bother with new cells. Get a gel cell like Troy said... Not necessarily more punch but lasts a hell of a long time before being recharged and cheaper as well.