IS your rear hub a single speed hub? Or is it a freehub with spacers and a single cog? If it's the latter then the conversion is pretty easy: get a rear derailleur of your choice (I vote XT) and a cluster to go with it (8 or 9 speed...10 makes it trickier). Up front if you're replacing cranks anyways then I say get a nice road or mountain crankarm set and mount a ring of your choice in the middle mount (maybe have your LBS help you with this). Then put one of these on each side of your ring (same size, one size bigger than your ring): http://www.bbgbashguard.com/

The trick will be, as you will see, whether your bottom bracket spindle is too long or not. If you've been running a single speed hub then you'll probably want a new BB with a shorter spindle. You can get the Truvativ one for $15.

Oh yeah, you'll need a shifter and cable, too. In my opinion the best way to go is just getting some cheap thumb shifters (friction) like these: http://www.ebikestop.com/items/004-LD1102

The Falcons are cheap and bombproof.

I understand your plight. I really enjoy a singlespeed, but there are certain times I wish I had a little range for speed on downhills and climbing...my legs get tired!