older macs are PowerPC processor based, and is almost a totally different way of doing things. for this reason, you can't run software compiled or written for PPC-based macs on Intel based macs, without it having to 'emulate' or translate the code into the approprite way of doing things.
think of it as having a conversation with a french guy with the aid of a translator. you can still hold a perfectly good conversation, it's just slower.
Photoshop has been proven to run quite happily on intel macs, but with a performance hit. apparently it's about the same as running photoshop on a 2-3 year old mac. when Adobe get around to recoding photoshop into whats called a Universal Binary (which would run on both PPC and Intel), the new version will run at the full speed of the Core Duo, which will be very fast. Adobe have said that CS3 will be the first Universal version of their applications, scheduled for next year. until then, photoshop will run reasonably well on your mac book.
Dual 1.8GHz G5 2GB, 1TB, Radeon 9600XT 128MB, 10.5
20" Apple Cinema Display + Dell 2005FPW 20" dual-head
iBook G3 700MHz 640MB, 40GB, Rage128 16MB, 10.4, dying battery