>On an aside. Over at the Fry's Electronics stores here in SoCal, I noticted that there is only a $20 price difference betwen the retail Win7 Professional edition and the Win7 Ultimate edition -- so we're talking about $290 for Professional and $310 for Ultimate. Hrm... IIRC Ultimate covers all the functionality of Professional, and includes some features useful for gaming and multimedia (some of these features are in Home Premium, but not available in Professional). The retail Professional comes in either 32-bit or 64-bit versions (i.e. you have to choose one or the other), whereas the Ultimate contains both the 32-bit and 64-bit versions. A $20 difference in something that's about $300 doesn't really represent much of a saving to opt for the cheaper one.
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>As for the OEM version -- it's about little over half the price of the retail editions -- though the difference between Professional and Ultimate is $40... Which then comes down to choosing betwen $160 and $200 -- so the difference in pricing between the two is relatively large, so it might make sense to "go cheap" if you know you'll never use the particular functionality. The other thing that seems to be the case -- the OEM package for Ultimate edition comes in either 32-bit or 64-bit (you have to choose -- it doesn't contain both).
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>One thing that I've also noticed in the licensing agreement for the OEM package. The WinXP the license allowed for two activations (not to say you could install on two separate computers, but does have provision to allow you to completely transfer the OS to another computer -- say in the case of a hardware upgrade, or replacement when motherboard or CPU dies on the original). The wording for the Win7 OEM package seems to indicate that there is ONLY ONE activativation allowed (meaning that it doesn't provide for the provision to completely transfer to another computer). The licensing agreement for the retail seems to have wording that's more along the lines of what the WinXP version had -- which did have a provision for (complete) transfer to another computer.
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>Some time between SP1 and SP2 release of XP, the wording on the OEM copy changed to drop the requirement of a hardware purchase to get the OEM release of the OS. Previous agreement specified that OEM copy was only to be available with purchase of computer hardware (likely intention was to specifically require purchase of an entire computer -- but wording was such that you could qualify by simply buying a peripheral -- like a $9 mouse -- which is what a lot of folks apparently did). I'll have to check if they changed number of activation however. Even if it did say that you were only allowed one activation only, apparently this restriction was rarely enforced. BTW, I did find out from more than one source -- Microsoft DOES enforce the limitation on OEM release of Win7 (which means, you could find yourself having to buy another copy, or pay Microsoft an additional fee to re-activate on another computer if the motherboard in your computer fails).
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>You can avoid having the activation limit by getting the Corporate bulk package for the OS (actually, you don't actually have to go through the activation process -- just install and you're ready to go). Of course, the only minor hitch with Corporate edition is that it's priced around the same as 100 individual copies -- and perhaps increased probability of an expensive software audit if Microsoft suspects you're doing fishy (i.e. giving away copies to folks aren't working for you).

Thanks for the detailed message. I bought Win 7 Professional Upgrade not because it cost less than Ultimate but I just didn't see much need for the two extra feature that Ultimate has compared to the Professional (from the list on the back of the box). I also have an unopened copy of XP Professional. So I plan to create 2 primary partitions on the hard drive. I will install XP on the first primary partition and then install Windows 7 into the second partition as a dual boot system. I will also create two logical partitions where my source/data resides.
The above configuration will allow me to use Win 7 and if necessary go back to XP or test on XP.