Cannondale makes the Stiffest and lightest frames (Trek is another good company but stay with their Made in USA Models which reach almost to Cannondale Standards). There are others ask your local bike dealer (Through remember he or she wants to sell the bikes they have in stock).
Now as a new bike rider for all practical purposes, I would go with the Cheapest Cannondale or Trek (Or other) bike you can get. You can upgrade components later, the single most important thing is to buy the best FRAME you can get. The stiffer the frame the easier it is to peddle and thus the greater enjoyment you will have in biking. AS you increase your biking you will see the better components and replace the cheap ones on your bike. You are NOT at the level to use the best so go with the best frame you can get.
Second, as to components I have Shimano XTR on my bike (I over-purchased). My sister has Shimano XTs on her bike (We both upgraded from lower level components). The XTR is generally twice the price of the XT, but is only marginally better (i.e. XTR NOT twice as good XT).
SRAM is an upcoming American component maker, it 9.0 series is almost as good as the Shimano XT. There are other component makers, but these are the two most common. Cannondale makes components in addition to Bike Frames, but Cannondale Compo nets are NOT as good as Shimano or SRAM.
Link to SRAM:
http://sram.com/en/Cannondale:
http://www.cannondale.com/bikes/index.htmlTrek:
http://www2.trekbikes.com/index_us.phpLink to Shimano:
http://bike.shimano.com/One last Comment, get a bike you like. If you want a Mountain Bike go buy one even if you never take the bike off the highway. Part of enjoying biking is being comfortable in your mind as while as in reality. Thus if you see yourself as going off road, buy that Mountain bike no matter what anyone else says another bike would be better.
Second, here are thread where this topic has been discussed before:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=324x1209http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=324x1217#1220