The discs are usually re-released with the SP updates after a while, so you may as well get it. It's literally nothing to ponder as the SP updates are just Windows updates you can download. They usually replace the older ones, so it "shouldn't" be more expensive (but typically they try and clear old inventory and offer slight discounts on the older ones, and if you don't mind spending time applying updates to save the few bucks, it's an option).
The real questions should be...
32-bit or 64-bit. I see little reason for going for 32-bit these days. It's one of those things you should only do if you know you "need" to. Otheriwse, go with 64-bit.
The other is Home Premium, Professional, or Ultimate. Unless you "need" Ultimate, forget it, and really, the same goes for Professional, but since it's not much more expensive (typically), some just spring for it. It won't make your games run any better or worse though. Unless you need the extra networking stuff or the XP mode of Professional, you can go with Home Premium.
Also, OEM vs retail. I would go with retail. OEM is cheaper, but it's not by much, so retail pays for itself by not restricting your PC overhauls. OEM is meant for when every dollar counts and you're building a PC that will never have it's platform/core changed out.
Lastly, Full versus Upgrade. If you have a copy of Windows XP or Windows Vista, you may as well get the upgrade as it's often way cheaper.
So, in my opinion, unless you know you need anything different, the basic/default choice would be a retail upgrade copy of Windows 7 Home Premium, and go with 64-bit (if you get the retail version, you get a product key that works for 32-bit and 64-bit, and both discs anyway).
Edited by Princess Garnet - 10/2/11 at 6:32am