Welcome to the macProVideo forums!
pre intel mac's are refered to as PPC and of course the new mac's are intel. Performance on the Intel mac's are extremely good, and better than the PPC, especially for Logic 8.
Most plug-ins are in Universal Binary format. This means they will work on PPC and Intel Mac's. However, a new OS is being released for Mac's, perhaps sometime before April next year, which is rumored to take real advantage of Multicore processors, but will only be compatible for Intel Mac's.
I don't think it's a good idea to buy a PPC G5 now as most plug-in manufacturers will begin to move towards supporting Intel only Mac's...
If you can stretch to it, I'd recommend getting an iMac or even the older model white macbook ($999), upgrading the ram and using an external display if you have one and an external HD for more space.
Logic's performance on the white macbook would be better than the G5 in some regards and almost matching it in other ways.
If that is not possible then look for a second-hand intel iMac or Intel MacPro depending on your requirements.
Logic is great! The Instruments and effects IMHO are excellent in general... But then I'm a little biased ;-)
Logic won't be able too use your VST's, it only uses AU's (audio Units). You don't mention which plug-ins you've got. For most plug-ins you buy you get the AU & VST in one.
There is a VST to AU wrapper available that allows you to use VST's in Logic, but I'm not sure if it'll work for PPC to Intel etc.
Let me know if you have any other questions, or need more help.