No One Lives Forever. Bargain game for the GOTY (I want the music CD, but I'm not buying it again), excellent single player, and quite long at that. Multiplayer isn't as good, but it's entertainment. Serious Sam's a blast for awhile, especially if you like coop with a friend, also a bargain. Heck I think they go for $10 already in some places..
As for a nice spanking new game to go along with it, RTCW is decent, but I'd wait for it to come down from the amazing $55 price tag. AVP2 is fun, but is a bit short and MP is quite crappy. Tribes 2 is a blast, I'm enjoying it a lot more than RTCW.
My suggestion: NOLF, Serious Sam, Tribes 2. Should come to a total of $50. Buy RTCW later.