After reading your reply to Deacon's thoughful question asking why you want to Dual Boot, it's clear you'll want 64 bit after you compare the difference.
For this and other reasons, your best approach is to unplug the HD with your old Windows 7, plug it's cable into the target HD, boot the Windows 7 DVD, select Custom Install, use Drive options to delete all partitions, Create new one(s) as you wish, format before installing Windows 7 to first partition which will be immediately after a 100mb System Reserved partition Windows 7 creates when formatting the HD.
Once you complete the install you can plug back in the other HD, set preferred one to boot first in BIOS setup, boot the other one using one-time BIOS BOot menu key at boot. This keeps the HD's independent so they can come and go as you please, whereas configuring the Windows Boot Manager interlocks them requiring surgery to remove one.
However if you want the Windows Dual Boot, install EasyBCD to the new install to add the old one which will make the old one easy to remove by just formatting when the time comes.