If this is your first MTB and you haven't been on a bike in while, I would be very hesitant to buying a bike on-line. Rule #1 in biking: make sure the bike fits. Rule #2: spend as much as you can. You could have a 6k full carbon bike that is a size too small and you won't enjoy it as much as the 1k bike that fits you like a charm. Getting a proper fit on the bike isn't something you can do on-line. As for rule 2, generally the more money you spend, the better bike (within reason). There is a huge jump in quality from a 300 dollar bike to 700 to 1,000 to 2,000 (for hardtails). Go to your local bike shop, tell them how much you have to spend and what type of riding you want to do. At the 700-1k price range, most bikes will be similar, so go to the shop that makes you feel most comfortable. As for components, anything Shimano LX (XTR, XT, LX from best to good) or SRAM X7 (X0, X9, X7) will work. However, don't overlook the quality of the wheelset and fork which is actually more important. Too many choices to get into, but a good shop should be able to describe this to you better.
Enjoy the new bike. 700 dollars is a good place to start. You will have a good mix of decent components that will take you where ever you can pedal it. Once you catch the biking bug, you can then upgrade parts or to a whole new bike.