I have had great luck with the Eason Havens, in 6 months of solid riding carrying my 200+lb. butt up and down lots of stuff they are still as true today as the day I got them. Very stiff but super light. They list for about 900.00 per set, but a couple of places have them for 525-550 shipped right now.
There is no wheel that will make you climb "steep" hills better than what you already have. Flat ground acceleration, momentum, mild / mod climbs could be easier with a super light wheelset. Steep climbs (granny gear stuff) will be so slow that rotational weight will be less of an issue than dead weight.
I'd say stick with what you have and work on the motor.
If you have 'itis and just want to upgrade for the sake of having some nice wheels...I've been extremely pleased with my Hope Pro II / DT EX5.1d wheelset from Universalcycles.com. You could go similar with a lighter 4.2d etc... rim and have a beautiful XC wheelset.
Best way to lose weight for that long epic ride would be to take a laxative and diuretic the morning of. Don't eat or drink the night before. Leave the water out of your pack...it's the heaviest part.