Mavic 321's rear, 717's front--stupid?

Hi, Matt here. I'm 200 lbs, and not the most skilled rider. I recently bought a Giant HT, and got two Sun Rim DS2's--of which the rear tire is already out of true. This is not necessarily the rim's fault, as I tried riding UP a short set (3) of stairs, however, I recently bought a Giant Trance frame and a new XT Component group (for $600-I'm in China so I'm already building up a new bike (I'm embarrassingly obsessive-compulsive about things I like to do). Anyhow, I don't foresee my skill level getting better at a lightening fast rate, so that, combined with the fact that I'm a heavier guy (200lbs), is confusing me as for a new rim set-up for the new Trance.

I'm more of a XC biker, but I like to jump and hop anything I can right now (which may not be much, but I have been hitting a 2' onto flat concrete, that's about the biggest so far), and since I'm moving to Bolivia next year, I'm hoping to do some free riding, and lite mountain riding. SO, (finally) here's my question: What do you think about a Mavic 321 rear (for strength) and a Mavic 717 front (for lightness?). Will the bike be useless for climbing? Will it be overkill for the stuff I do usually? (free riding, aggressive XC)...
Anyhow, I'm worried about having a 28lb+ bike with the Trance, and was wondering if the rolling weight of the 321 on the rear would REALLY slow me down. Any thoughts on what this semi-Clyde, newbie should do about a wheelset?

Thanks,
Matt