Warning, Warning -- the 4.0 slows down the 7750 device really dramatically. Make sure you remove all your third party software, clear your browser cache, and maybe uninstall brickbreaker (if you have it), before upgrading. The 7750 is the slowest BlackBerry model that ever came out.
The model 7250 is 5 times faster and the model 8700 is 10 times faster. BlackBerry Benchmark Thread