You're on the right track with the Orange Box and Bioshock. Being as those were the first two games you listed, I take it you are a shooter fan, so yeah, I'd definitely say go for Left 4 Dead or Left 4 Dead 2 (over all, I like L4D better, but L4D2 has some pretty cool additions). Those games are a lot of fun, but they really shine the most when you play them online with friends (I've had some fun games with strangers too), otherwise they wear thin pretty quick.
Arkham Asylum looks really awesome. I have still never played it, but I have only heard good things about it.
Braid is absolutely fantastic, a really fun change of pace.
Bioshock 2 was released today, I've just spent the last hour playing it and it definitely lives up to my own personally expectations. Of course, wouldn't recommend it until well after you beat the first one.
If you liked Resident Evil 4, then I'd definitely recommend Resident Evil 5. If you never played Resident Evil 4, then I'd highly recommend it.
It seems like these days Halo is equated with downing copious amounts of Bud Lite and going four wheeling (i.e. very redneck), for some reason. So, at the risk of sounding extremely low class, I actually really enjoyed Halo 3 and would recommend it. It feels a lot different from other shooters. Rather than just blasting through things, you often have to stop and plan out an attack strategy, otherwise you're doomed to fail again and again.
Fallout 3 has earned a permanent spot on my all time favorite list. I've gotten every achievement in that game, and yet there is still so much of it I haven't seen, so many characters my character has never met, and so many locations I have never discovered. The GOTY edition comes with all the expansions, which are definitely worth getting.