I'll definitely buy something new, because it's been about a year since last time...
Power of Faerun seemed to me the most interesting supplement of 2006, although I'd like to see more reviews about it. I was thinking this potentially has very useful information about high-level play (especially downtime and campaign building), and not necessary very related to Faerun.
Dungeon Master's Guide II could be useful too, especially since I liked Unearthed Arcana quite a bit, but I'm still undecided. PHB2 instead seemed bad for me.
Then I was considering to try one of the environmental books, this series is to me possibly the only interesting one among all 3.5 series. Eventually it will depend how much can fit in my 3.0 games with minimal adjustments. And also I want adventure material most, and as few character stuff as possible. I guess I would need to check the books out at the FLGS before buying.
From reviews, Stormwrack seems more about environment and less about characters, compared to Frostburn and Sandstorm, which would be best for me, but also "underwater" is not very much an attractive environment to me...
Cityscape sounded great when first talked about, then I heard it's too much about building a town than actually let a town run... probably not more useful than the old Stronghold Builder's Guidebook.
Then maybe 1-2 adventures since they aren't a big deal IMO whether they're 3.0 or 3.5 really, and the new ones coming out since last year seem to be very successful.
Probably nothing of what is scheduled for 2007, except maybe more adventures.