Just a few points of caution from one who's fought the timing belt monster recently:
1) Strongly suggest you go to a Lexus dealer and buy "only" the OEM timing belt, not one from NAPA or elsewhere, even if it's a Gates or other well known brand. For some reason, the aftermarket timing belts seem to be just a bit longer and can be difficult to tension.
2) When putting the belt on, one of the tricks that isn't stressed enough in the FSM (or why) is making sure when you install the belt, that all the "slack" is over where the tensioner is located. If this isn't done correctly, the three marks may look like they line up correctly on the pulleys, but it still may be slightly out of time. Sounds weird, but it can happen, especially with an aftermarket belt which is just a bit longer than it should be.
Lexus/Toyota did us a favor by putting the three marks on the timing belt, and they are definitely a big aid to getting the belt on quickly, but if you have an aftermarket belt that doesn't fit exactly right, those three marks can be right on the money, but the timing can still be off.
I currently have a Gates aftermarket belt on my 1UZ, and the only way we were able to keep it timed right, was to insert a 3/16" shim under the tensioner:
http://www.lextreme.com/forums/showpost.php?p=77137&postcount=80
Also have a look at post 78 which shows the hazards of running a worn out, or too long timing belt. And post 74 is where I described the problem.