1. You can purchase the retail upgrade package, and perform either a fresh install, or a dual boot, or simply upgrade your existing system.
If you choose the retail upgrade version, and a fresh install, you will need to insert your old version CD at some point to prove that you qualify for the upgrade.
2. The choice of OEM vs. retail somewhat depends on how you interpret the ELU. Technically, the OEM version may only be installed with the associated hardware.
On a practical basis, thereis not as yet, hardware specific versions of the install CD.
The OEM versions of Windows often inclusde the Plus! package. Or other add-ons.
I voted with my pocketbook, and choose the retail upgrade version. I bought it when XP came out, and took advantage of all the bundled freebies that my local retailer offered.
If I was going to set up a new machine, I would order an OEM version. I have several local retailers that will bundle an OS with selected hardware purchases. So, you can save enough to get a free hard drive, or processor upgrade. That way, as long as you install the hardware in the new system, you are in compliance with the ELU.
You do have to watch the OEM versions of non-OS software. Many of them are SE (special edition) or LE (light edition) so they have limited functions.