You can get an excellent entry-level mountainbike from a GOOD local bike shop.
I cannot be sure, but you should be able to get a good one for ~$400. Fuji, KHS, Giant, Jamis, Trek, Motobecane, and Raleigh tend to have good bang-for-the-buck.
Shop around, there are some good deals out there on last-year's bikes.
A few examples:
http://www.mtbr.com/reviews/xc_hardtail/
http://www.mtbr.com/reviews/XC_Hardtail/product_123242.shtml
http://www.mtbr.com/reviews/XC_Hardtail/product_123244.shtml
http://www.mtbr.com/reviews/XC_Hardtail/product_123258.shtml
http://www.mtbr.com/reviews/XC_Hardtail/product_123323.shtml
I will say that there is nothing wrong with a department store bike, overall. But, they tend to be very cheaply made, HEAVY, and the components are often proprietary--you cannot easily get replacements! Further, some parts are totally non-standard.
Major brands found at a bike shop will use more standard and easily attainable parts.
I recommend a mountainbike over any other because it is the most versatile. Try taking a roadbike off-road and see what I mean! (Yeah, I know, cyclocross...)
Make sure you get the proper size!!!!!
A good bike shop will help you do this.
They will also help you set up the bike properly.
Get a helmet and wear it!!!
A good one can be had for ~$20.
Edit: sorry for the rambling post, no coffee onboard yet!
Suggestions:
1. Determine your budget.
2. Go to a GOOD bike shop, look around. Tell them your needs.
3. They will fit you to a proper bike.
4. You will also need a helmet, bottle cages and bottles, saddle bag, simple toolkit (with tire levers, and possibly a chain tool), mini-frame pump, spare tube and tire patches. Believe me, you do not want to be stranded 20 miles from home with a busted bike.
Take care,
Bob