Battlefield 1942. and dl the Desert Combat mod too. amazing. you can also try the Dark Watch Clan server around 21.00-01.00 EST weeknights and weekends. (www.darkwatchclan.com)
I second/third Deus Ex. It's still a FPS, but with a good story line that helps suck you in much like Halo.
Another great game is The Sum of All Fears. This uses the Ghost Recon engine, but the missions are all anti-terrorist usually in buildings. A big change from looking for small movement across fields (and a lot more fun). I found myself using different loadouts and trying different techniques for the fun of it. Only loosely based on the movie.
Both of these games should be under $15 so they are well-worth the money.
Don't know why, but I take back my comments about SOF2. I started it again, and the soldiers in the first level are dying with just a couple of shots now. I don't know what the problem was before. Still can't say I'm crazy about this game yet. The Prague levels are kind of boring, and I have to wonder about the wisdom of starting a game with that long a flashback sequence. Maybe others who have played it to completion can say whether they liked the solo play or not.