Get one of these:
http://www.bikesdirect.com/products/...iberty_cxd.htm
Or if you're willing to spend a little more, one of these:
http://www.bikesdirect.com/products/...ane/outlaw.htm
I think the sizing is different between the two. Either will satisfy all of your requirements and feel much quicker. I don't think putting money into changing the gearing on your MTB is a good investment unless you really know how to get deals on parts and do the work yourself. I think MTB's can be made to be the most bombproof commuters, but you're looking to do longer stuff on the weekends. I just don't see being able to get the bike good for that without a lot of money invested. Good luck.