The problem with analizing the trend of TX is that there's been a George Bush of TX on the GOP ticket for 6 of the last 7 elections, and then there was LBJ on the Democratic ticket in the '60s. Some of the shift from 1964 to 2004 can be explained by home states.
When LBJ was on the national ticket in the 1960s, the Republican Party in Texas was barely functional.
When George W. Bush was on the ticket in 2000 and 2004, the Democratic Party in Texas has been barely functional.
If in 2008, we were to start off with a standard liberal Democrat and a standard conservative Republican in Texas, in a best case scenario you will get numbers of roughly 55% for Republicans and 45% for Democrats (the statewide race I would compare to that is Cornyn v. Kirk Senate 2002). Dole in 1996 (won the state with 48%) would have probably gotten 53%-54% in Texas without Perot, and the state has trended slightly Republican since then (my best guess).
A Southern Democrat on the ticket could add a couple of points his way, but in order to have a chance to win, you would probably need a pretty liberal Republican candidate added on top.
And all of this stuff is not set in stone either. A Southern conservative Democrat will have certain problems in the Northeast. A more moderate, outside the South Republican would have certain problems in the South.
All in all, I would pay attention to Al's maps. They tell the biggest story.
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