My lowest amount of money spent for a good game was also Orange Box.
I also got Assassin's Creed for free (buy 2 get 1 free deal at GameStop).
Team Fortress 2, being an online shooter, can be a bit overwhelming at first as far as difficulty, but not nearly as much as others due to the fact that it's team-based. It'll still take a bit before you start winning one-on-one firefights. The game is hella-fun, the only online shooter I play 'cause others feel like a step down.
One thing I'd suggest for you:
You're on NewGrounds, so I assume you've played Portal: The Flash Version. If not, DO SO NOW. Afterwards, and after beating Portal, the people that made Portal Flash Version actually converted their 40 levels to 3d (and made a few new ones) and put it together like a whole new Portal singleplayer campaign! It's really awesome, and you can download it for free here.
Congrats on Orange Box. I'm a GameStop employee and if anyone ever asks for a good game on 360, PS3, or PC and is even remotely interested in shooters, it's the first thing I point out.