Ghost Recon = Best, probably most realisitc FPS I've seen. It's $20 for the original game, and about $40 with two great expansion packs added on (a desert one and a tropical one). Guns are all great, gameplay great, especially online you can get usually a game with at least 18 in it (up to 36, but hard to find one with so many). Graphics pretty good too. How about this, for $30 you can buy the $40 pack and I'll buy the tropic add-on for $10... for some reason they don't sell it seperately anymore (more $$$ from lazies like me who've waited this long to buy it). If realism in a tactical FPS is what you want, definately, definately, definately get this game (all 3 and sell 1... remember

). Get all-seeing eye or something like that though, the online network it comes with blows. Oh yes, and if you're just in it for single player, there's about 15 missions in the first one, another 8 or so in the second, and a few more I'd suppose in the third, and they're difficult as hell on expert.
If you want more arcade style, I always liked the Delta Force series, not much on graphics or realism, but you can get like the first 3 of them for dirt cheap and online play is up to 50 players at a time, and the sniping aspect is great (unlimited level sizes and great maps for sniping for 500m-1.5km), there's not much weapon choice though, I think the first one isn't playable online anymore either, and single player blows.