Tiger scales happily off a video card to the CPU if your card does not support Core Image, but Core Image is not used that much throughout the OS, only in certain apps and only for eye candy. For instance, on a G3 machine, the splash effect on dashboard works, but it is choppy. There was more info about this on Appleinsider.
Early builds of Tiger were known to corrupt hard drives, as were early builds of Panther and 10.0.x for that matter. You are not necessarily safe if you run Tiger on an external hard drive with the main hard drive mounted.
Not to spoil your party or anything, but I would just wait until Tiger is released. We developers generally have dedicated machines for those early builds so we do not screw things up. Some apps are broken, the operating system crashes, things get corrupted and if that is your cup of tea, by all means still the OS and run it. Otherwise, just leave it up to us, we have to reformat every two weeks anyway.