I'm not sure of the speed of XP running under 7 in XP mode, but you can install both OS's and dual boot. When the public beta of 7 first came out, I downloaded and installed it on my XP machine. Then when I would boot the machine the boot manager would come up and ask me which OS I wanted to boot to.
I got the impression that if you installed 7 along side an already installed XP that it would work this way. I'm not sure if you can install XP on a machine already running 7 and get it to work completely this way. Of course you can take a machine already running 7, install a second hard drive, and then install XP on the second hard drive. Then you can switch which hard drive boots first in the bios and run it that way. I had originally intended to install my 7 that way but when I got it I was lazy and just installed both on the same hard drive.