Investing in a proper (edit: very good quality) office chair for my development rig was certainly one of the best investments I made. I don't know if you've seen it, but if not I suggest to take a look at http://www.codinghorror.com/blog/2008/07/investing-in-a-qual...
I used to have this one (the older version, with better back support) http://www.ikea.com/fr/fr/catalog/products/00146677/
But with my new flat, I still dont have much furniture and use this (shame on me I know :)). http://www.ikea.com/fr/fr/catalog/products/50221711/
I might search for a really nice chair in several months though. I would probably go for something like this : http://www.ikea.com/fr/fr/catalog/products/S19896843/.
In my opinion, best back support is simply not to stay hours without getting up on the same chair. The yoga position keeps your (or at least mine) back automatically in a correct position.