You shouldn't need to swap disks in the BIOS. While I was running Win 7 Beta and XP Pro in parallel, I set the machine up to ask each time which I wanted it to start. I allowed 10 seconds for me to choose, then it would start XP Pro.
There are three hard drives in the case, and by then the boot partition wasn't partition 0 on drive 0, but it all worked.
HOWEVER ... to reinforce what x213a says...
After the first week with Win 7, I never used the XP Pro boot setup again. I could access the datafiles (Word, etc) from either boot, and very quickly decided I far preferred Win 7. I switched over the start routine to 5 seconds and Win 7.
Now that I have the released version of Win 7, I've removed XP Pro altogether and have other stuff on that partition.
If you really want to do dual boot, leave the XP Pro drive in there, create your new empty partition for Win 7, and when you install 7, tell it where to install to (ie the empty partition, and not to overwrite the XP).
At some point in the setup, it will ask you if you want it to set up a dual boot.