It all depends on DeLay
The race to be DeLay's successor all depends on DeLay.
Obviously, the date of the race will affect who can win it. For example, DeLay probably doesn't want Tom Campbell to take his seat. Campbell's likelihood of winning decreases the longer the wait until a special, because his advantage right now is fresh name ID. In a longer campaign, that advantage decreases. [Edit to note that Campbell would not be considered the frontrunner at this point in time, so don't construe my words that way.]
If DeLay resigns at certain times, then it might make it difficult for some candidates to run, especially some state legislators.
So who jumps in this race will in some ways depend on when DeLay announces that he will resign. DeLay will probably feel some pressure to let people know when they can expect a resignation.
In the next few days, we'll know lots more about how this race will shape up.
(Also, if you've been coming to this page, I've been updating the posts below.)