Just a thought, why not try it with Win 7 64bit, install all your applications and see if that run fine on it? Then decide if you want to dual boot. I was about seceptical about going to 64Bit, but going forward, everything from Windows will most likely go to 64Bit anyway so you might as well make the jump. I've had 64bit running for awhile now and all my applications I've had in 32Bit, I've moved across and they work fine.
You can also try running the application in Compatibility mode (same as Vista) if applications don't run in 64bit properly. I've only had to do that for one program.
I'm just thinking it would probably be a lot easier than having to reboot your machine all the time to switch to Win XP from Win 7 if you wanted to run something in Win XP. Win XP Mode in Win 7 does work as well - but I haven't had to use it.
As arges86 said install XP first, but remember as you are upgrading to Windows 7 Pro that allows you run XP mode which negates the need for a dual boot with XP.
If you want to keep XP on a seperate drive there is no need to reinstall it, just install W7 on the other drive.
First, I would recommend that you disconnect one HDD and install operating system for that drive - DOES NOT matter if you do xp, or windows 7 first. Once that operating system is installed and up and running then DISCONNECT that HDD and install/connect the 2nd HDD and install that operating sytem. Once that is up and running you can reconnect the 2nd HDD with That operating system on it.
Instead of a software Boot Manager you would hit the Key (On my Gigabyte MB it is F12) during the Post. This brings up a menu of which drive to boot from ( will not affect Boot prioity set in Bios). If your boot prioity is set to the Drive with Win 7, then that drive will come up as "C-drive" Hitting ie F12 and selecting the HDD with XP on it will now boot to XP and XP drive will become "C-drive" You can still access any files on the Win 7 drive (Will be "D-drive, or E depending on additional partitions you created).
Big Plus, the two operating system are independent of each other and not have to worry about a screw-up on one drive messing up booting to the 2nd drive.
Above method is what I used to triple boot to Vista, XP, and Win 7 RC. (used 6 HDDs, 3 pairs in raid0 - Never a had a problem I even used say the "Vista and the Win 7 RC HDDs to store files created in XP and Visa versa. Only thing is noting drive letter changes when changing which OP system I booted to.
I've decided to install Windows 7 first and see how the XP mode works. If it doesn't work to my as I thought is should, I'll do the dual boot. All the informatin that was been provided in this thread has been very informative and will be useful if needed.