We'll obviously cost isn't an issue so why not just get Ultimate and be done with it. He might have suggested Pro because it allows you to run XP mode (which isn't for games) or something but you're right Home Premium is what you need.
Is there a difference between a windows being OEM and not OEM?
Yes, but it is only support. When buying an OEM version, Microsoft has this disclaimer.
Use of this OEM System Builder Channel software is subject to the terms of the Microsoft OEM System Builder License. This software is intended for pre-installation on a new personal computer for resale. This OEM System Builder Channel software requires the assembler to provide end user support for the Windows software and cannot be transferred to another computer once it is installed. To acquire Windows software with support provided by Microsoft please see our full package "Retail" product offerings.
So if you know what you are doing and don't need to contact Microsoft for support, then get the OEM version to save money. The retail version is only for people who don't know how to troubleshoot software problems or need to transfer the license to another computer if they have major hardware failures or they upgrade their computer too much and can't pass Windows Activation.
And either way you go, you need x64 because of the amount of RAM you are using. Anything over 4GB needs a 64 bit OS to work. You don't want Starter Edition or Home Basic cause half of the most used features are disabled. Some versions you can't even change your desktop wallpaper. Home Premium is geared for home users who want to play games, and use a lot of internet apps like music players and watching videos. The Professional Edition just gives you some more business related enhancements and the Ultimate Version gives you all features.
I myself, always use Professional or Ultimate because my wife is Chinese and sometimes has trouble reading some of the English in the system settings, and with Pro or Ultimate versions you can download language packs that will make her user account Chinese while mine is English.
If you don't need multilingual support or some advanced networking tools, just get Home Premium!
I've tried the XP mode. I think it sucks. It is better to download the free Microsoft VirtualPC and install a regular copy of XP. When using the XP mode, you can't even shut the VM down or restart when installing software on the VM. You have to go through 2 or 3 steps to get it to shut down or restart. If you click the Start button in XP mode all you can do is set it to sleep or save state. I think whoever set it that way was stupid. If you can't shut down a VM from inside the VM, it's just plain weird.
Also when using the free VirtualPC or Sun Virtualbox, you have more control over what hardware the host can share with the guest. The XP mode integration features don't work right half the time.
If you really need XP for some legacy reasons, just dual boot it from an old hard drive. Even if you have more than 4GB of RAM, it'll still boot and let you use 3.2GB anyways. Or use Sun Virtualbox, it works better than both VirtualPC or XP mode.