Yeah it's going to be a bit of a problem. What you're "supposed" to do is buy Office via Microsoft's Volume Licensing plan, in which case you get a single product key that will work for multiple installs and bypasses activation. The same is true for Windows itself.
That said, you can try something like this: http://www.mydigitallife.info/2007/01/12/how-to-reset-or-change-microsoft-office-2007-product-license-key-or-volume-license-key-vlk/
Personally I haven't had good luck messing with product keys recently, especially with OEM versions of Microsoft software.
I buy machines in small quantities as needed, so I don't typically do volume licensing for Windows (although I do use it for Office). What I do is maintain a large archive of restore disks. It's a pain, but not a big enough pain to put much time into. Another option is store images of each machine when it's setup and ready to go. This will cost in disk space and time (since you probably won't maintain all of the images), but it will save time over having to do the restore/reinstall from source media.