If you're looking into expanding your PC collection and testing your taste, you cant go wrong picking up some FREE games, especially considering your limited budget.
Of course, many free games are garbage. That said, I have a few I can more than recommend:
Tribes 2 - Literally THE BEST team based shooter of all time, IMO. Think team based Quake, with jet-packs, vehicles, and base assets. The official servers for T2 have been shut down but you can snag the game and run it for free on new fan-run servers. www.tribesnext.com
Global Agenda - Jetpack-enabled Third person shooter MMO hybrid, you can find it on steam. I can personally offer you an agency, help in the game, and a coupon to get "elite agent" status for 30% off if you later decide it is worth it to you. Add me to your friend list in-game as ZeroMMXZ.
Anarchy Online - The best free true mmorpg out there, imo. Think WoW+Everquest, only bigger, and in space and in the future... The only downside is it is a very old game, and the graphics show it. It is also very complicated (as is Tribes 2 for that matter) so feel free to ask people in game (or shoot me a PM) if you need any help.
Alien Swarm and Team Fortress 2 are both free on steam. While I'm not in love with either, it wouldnt hurt to grab them. I'm sure you know all about TF2 by now, and alien swarm is a team based top-down co-op shooter.
Legions Overdrive - Fan-run version of Legions: Fallen Empire, a game from the now dead instantaction.com. It is essentially a stripped down, streamlined version of Tribes which focuses on maneuverability and small-scale battles. A quick google search should find you the download.
Now, as for spending that amazon money, id grab Just Cause 2, or the Guild Wars Trilogy