Something that hasn't been mentioned since the original poster is upgrading from a PowerPC Mac: If you're still using any applications that run on the Classic environment, you won't be able to run them on the Intel Mac unless you use an emulator like
SheepShaver which emulates a PowerPC Macintosh and needs an installation of OS 9 or earlier (similar scenario to something like VirtualPC but it emulates a Mac instead).
Otherwise, all PowerPC-based Mac OS X apps should run fine, albeit under Rosetta on the Intel Macs.