1. The most important thing about buying a new
bike is to make sure it fits. The only way you'll know if the bike is right
for you is to size up the bike and make sure that the bike's geometry matches
your body's geometry. Ask questions and do some research.
2. If possible, try to find a shop that will
let you demo the bike on real dirt. Five minutes in a parking lot won't cut
it. You wouldn't buy a car without a real world test drive, and a bike should
be no different.
3. Don't belive the hype. Just because
your favorite rider or best friend rides a certain bike, that doesn't mean
that's the best one for you. Have an open mind and be realistic about your
needs and ability.
Most people do use two different tires. Even if you get the same tire, many of them are designed specifically either for the front or the back so even if you buy the same brand and model of tire, they may be different.
The forums and reviews are rife with examples of running brand x model y width z up front and brand a model b width c in the rear. The reasons vary, but trail conditions and riding styles seem to be the main factors.
Need to make sure the frame can accept the width of your new one.