The Texas Secretary of State's website keeps track of the numbers of voters each day of the early voting period. Voter turnout this year is high. Really high. See the figures below, created by me from data on the secretary of state's website.
One difference between the two years, though, is that by this point in 2004 the major parties had their official or de facto nominees. Another difference between 2004 and 2008? So far in 2008 democrats are outvoting republicans at an overall rate almost three-to-one. Democrats are even outvoting republicans in generally republican counties like Collin, Denton, Fort Bend, Tarrant, and Williamson (though it's not an apples-to-apples comparison, since the democratic race is closer and more dramatic than the republican).
The biggest turnout in Texas for a presidential election was 1988, when 2.7 million people voted. Secretary of State Phil King is predicting that in 2008 3.3 million Texans will vote.