Full Spectrum Warrior, Rainbow 6, and Ghost Recon 2. Halo2 is a great FPS, but I prefer "killing" earthbound combatants more than aliens. Also, take a look at Crusader: Kingdom Under Fire for a mix of tactical and button mashing hack-n-slash. I'm looking forward to Mercenaries in January and Brothers in Arms in March. They both look like they'll be good.
Sega's ESPN Series. I have NFL 2k5 and NHL 2k5, they're both awesome. Also, if you like boxing, EA's Fight Night is good.
Project Gotham Racing 2 or Need For Speed2: Underground
I dunno, but check out this site for game reviews.
I buy and trade-in games at GameStop all the time, so my collection is usually rotating. Lately I've been playing Ghost Recon2 and NHL 2k5 a lot.
Just a couple of other suggestions, take a look at subscribing to the Official Xbox Magazine. They include a demo disc in each issue, and I've found some real gems in there. Even more importantly, I've played demos of games that I planned on buying only to play it for a few minutes and realized it sucks. That magazine has saved me a lot of money.
Lastly, if you have broadband, you MUST get xbox Live (only $50/year). But even if you don't get Live, prepare to have your wife pissed that you're spending too much time playing games! Have fun, and enjoy!