Most of the major applications are universal and will run on both. In CS4 all the apps will work except for their video editing package which will nly run on Intel based Macs. I believe that the OS to come Snow Leopard?? wont support PowerPCs.
I have a Quad core G5 2.5 which could run another 4 years with the apps I use, but I will probably end up getting a new MAcPro because I dont want to get left behind, and is part and parcel of apples evil plan.
Mike