Do yourself a favor and get an AF longer zoom, and the best price/performance IMO is the Sigma 70-300mm APO DG Macro (1:2) (around USD 200)
The non APO from sigma are not very good from what I've read (more CAs)
Dunno about the Tammy and Tokina but IMO the longer reach, macro and AF of The Sigma 70-300mm are very good value for the extra 150 USD. One sign for me that it is a good lens is that you seldomly see it going up for sale, that means people are holding on to it.
Edited after seeing Adam's post: you have also the Pentax 55-300 which is a very good lens for the money (around USD 250) and the Tamron 75-300 (around 200) which is slightly inferior to the Sigma from what I've read. Look-up that link
If you go cheaper you'll regret 2-3 months later. My 2 cents.