1st one is the full version, which means you can install it on a PC with no previous version of windows currently installed. It is also retail, which means it won't be tied to the first motherboard you install it to. You can install it to a different PC if you wish, as long as you un-install it from the other PC first.
2nd one is just the upgrade version, you need to have a previous version of Windows installed on your PC and just upgrade that to Windows 7.
3rd one is also the full version, but it is OEM. It basically means it will be stuck to the first PC you install it on. It's not mostly the case though, a lot of people are able to install OEM copies on a different PC so it's like a retail copy but cheaper
There is nothing stopping you from installing an OEM copy on multiple computers, but you're breaking the license agreement by doing it.
As for Ultimate being worth the extra cash, take a close look at the feature set and decide whether you need what it has to offer. Of course, you can always use the Windows Anytime Upgrade feature to move up to Ultimate later if you find you actually do need a particular feature, but it's better to decide now and go with a lower version if you're not sure, than to go all out and get Ultimate, and possibly regret it later.
Basically, don't rush out and get it if you're not sure. Take your time and make an informed purchase.