I would definitely stay with a HT. It will be a better 'all around' bike for XC and other stuff. As for what to look for when buying... make sure that the frame and drivetrain are good quality. Of course that brings up another question... what kind of XC riding are you planning (hardcore singletrack, two track, etc.)? This will determine how much you should be prepared to spend on a bike. For easy trail riding with only the occasional jaunt into the single track, about $500-$700 can buy you a nice bike that will take a decent amount of abuse. If you are going to do more riding on techincal & difficult single track, you will want to look towards the $800+ range. If you look at bikes in both price ranges and pay attention to the frames and components you can get a feel for what I am talking about. For example, lets say you want to stick to the hard-core single track riding... dont buy a bike with anything less than an XT or XTR rear derailleur, and an LX or XT front derailleur and crankset (or equivalent). These components get the heck beat out of them in single track.. constantly shifting up and down the gear range. Next is the frame... Depending on your weight & size you will have to decide which frame material to use. I'm about 235lbs and 6'2" so aluminum is my frame of choice. Chromoly, TI and carbon are a little too flexy for my liking. Then there is the other stuff like wheels, fork, etc. Again, your size can play an important role here... I had a great set of Bontrager BCX rims on my old Gary Fisher Supercaliber and ended up tweaking them and breaking spokes all over the place. I had to get a higher gauge spoke to alleviate that until I got my latest bike which came with Mavic CrossMax rims... indestructible if you ask me.
Another important thing to look at is how the bike feels.... there are a lot of different geometries and sizes between the many bike manufacturers. Go to your local shop and take some for a spin in the parking lot. Pay attention to how each of them handle. Some are sluggish to turn, some are very flickable and maneuverable. Check out the different frame materials and see how they are different in handling.... try getting out of the saddle and cranking on a chromoly bike, then switch to an aluminum bike. Sizing too is important. Get a bike that you feel comfortable on... not too bunched up... not too stretched out. Again, your desired style of riding will come into play here. Tight single track is usually easier to negotiate on a smaller frame because you can move around on it easier... the opposite on long rolling two track.
Hope some of this helps you out. Let me know if you need some clarification on any of it.
