Honestly, don't go full suspension. It's not worth it at that price range. You don't need full sus. to go off of three foot drops. Trust me. Buy a nice hardtail and upgrade when you have the money. The wal-mart bike is going to be so heavy I'd be surprised if you could bunny hop it high enough to get onto a picnic table. It's not worth upgrading the wal-mart bike. Period. The frames are heavy, badly built, badly welded and will probably break shortly. Not to mention that EVERY component on it is crap.

Buy a low end brand name bike for around $500-$600. (Your price range for upgrading the wal-mart bike.) It will be plenty of a bike to do whatever you want. And it will come with low end, but still name brand reliable components.

Raleigh Bicycles - Tokul 1

The tokul is an extremely solid frame for future upgrading and will suit your needs perfectly now.

Take advice from someone who regularly works on both wal-mart bikes and nicer bikes. You don't want a wal-mart bike. Especially if you want it to EVER run/ride correctly. Wal-mart bikes are extremely difficult to get working/shifting perfectly.

Don't believe me? Go take a wal-mart bike for a test ride. Immediately go to a bike store and take their CHEAPEST bike for a ride. It'll be a world of difference.

EDIT: For the record, I would not, under any circumstances, take a wal-mart bike down stairs. I'd be afraid it'd break in half, and it probably will. Maybe not immediately, but it will eventually. (And by that, I mean within a few months if you do it consistently.) Where as a cheap name brand bike will last you forever provided you keep up on maintenance.