Probably, the most bang for the buck game that my friends and I have found is the orignal Ghost Recon series. You can get the core game and 2 expansions for under $20 and there is a massive mod community that's still supporting it. After you install some hi-rez textures, the game still looks pretty good and you can't beat the weapon packs.
FEAR is fun and creepy, but kinda easy and a touch repepitive
GRAW isn't as good as the original, and I've had some technical issues with it from the start
BF2 bah!
Rainbow 6 LV is pretty impressive. The originals were quite complex and I quickly lost interest, just wasn't my deal. This new one though, I believe they've got the formula just right. The AI seems to be pretty smart, both for and against you. The squad movement is simple, but it's highly interpretive, so your squadies do what you want. The cover system works really well, becomes second nature quite quickly. Plus, there's a decent amount of environment interaction.
A friend of mines one of the beta guys on ArmA and he's been keeping me filled in on the progress, looks good. I got the demo, and I wasn't bowled over. It looked like a retextured version of OFP, but a bit more streamlined.
There's always OFP(Operation Flashpoint), you can pick that up super cheap and it should keep anyone entertained once you remap all the keys.
One game series that never got the recognition it should have was the Hidden & Dangerous series. The 2nd one can be had for $20 with an expansion pack, and for a WW2 game, it rocks. Player models are quite good, to the point you can shoot peoples helmets off, and the guns out of their hands. Give it a look if you haven't before.