Running Samsung 1TB disk (Can't remember model) on Win7 with Ubuntu in VirtualBox, and Firefox, Winamp, iTunes etc open most of the time.
I didn't notice too much performance degradation during the use of apps after TrueCrypting the drive, but noticed a significant change in the loading time of apps, ie. when the drive is loading most of the prog. into memory.
I do have my games on a non-truecrypted drive, along with some other larger programs like Photoshop and Sony Vegas, but when Win7 is running from the encrypted drive, and you have all the programs already loaded into memory, I saw very very little performance difference, yet I do expect that this would be different for running games.