You can run a 32-bit OS on a 64-bit system. You use the same 32-bit drivers, too, not the 64-bit drivers. Your drivers need to match your OS (look at the section labeled "Can I run 64-bit programs on a 32-bit computer? "). Even though this link talks about 32 and 64-bit versions of Vista, it's still applicable to what you're doing.
You will almost always get a blue screen on boot when you install a drive with an OS on it from another machine (unless you change a drive from one clone to another). This is because the proper drivers for the new machine are not installed on the old machine's hard drive. To use XP on your new system, you're going to have to do a reload and to boot from one or the other, and then you will have a "dual boot" system even though you're using two separate drives. I'm not 100% sure about this because I've never tested this myself, though I'm pretty confident that you can't just select the drive in BIOS - you have to use a boot manager. I may be wrong on that statement, because I am not one who generally does dual booting, but AFAIK, you need to have a boot manager with two OS's, even if they're on two different hard drives.
FWIW, I had Vista Ultimate 64-bit and XP Pro 64-bit on my computer once (I removed the XP drive because I needed it in another system), and I used Easy BCD to create the dual booth with independent hard drives. There's a good bit of information out there, especially on their message board. Easy BCD will alter your bootsector on your primary drive (which I imagine will be your Windows 7 drive), so if you decide to uninstall the XP drive at some point, you will be prompted for the
Microsoft is conducting an online survey to understand your opinion of the Technet Web site. If you choose to participate, the online survey will be presented to you when you leave the Technet Web site.