For a tight budget, also look at your local Craigslist. You can find great deals on used gear as people upgrade.
I'd steer away from HTIB stuff. If I had a dollar for all the people I know who bought one of those kits and regretted it a year later when they wanted to upgrade part of the system and ran into problems with proprietary connectors and component arrangements (like surround speaker amps inside the subwoofer and crap like that), well, I'd go out to lunch!
If you are absolutely limited to your Best Buy gift certificates, look at getting a lower end receiver (Yamaha and Onkyo, as mentioned above, are great choices) and a nice pair of bookshelf speakers. A decent stereo pair should sound better than a mediocre 5.1 surround system.
Whenever you come up with some upgrade money, just start adding more speakers. Start with a subwoofer, then maybe a bigger pair of stereo speakers (moving your first speaker pair to the back as surrounds), then add a center channel at some point. After that, you'll probably be ready for a bigger, more powerful receiver. I assembled my 9.1 system over several years, in the same way, bit by bit.