First choice is easy - Vista.
It's the current OS, you wouldn't go and buy a MAC with a 2yr old OS installed, so don't do the same with a Windows environment.
As for OEM Vs Retail.
From what you've described then a Retail license is what you require.
The retail license allows unlimited transfers between "devices" for the lifetime of the product (you define the lifetime, not MS).
So you can install it on one machine now, move it to another in a few months, upgrade that a few months later and although you might occasionally need to re-activate over the phone you will remain license legal.
An OEM license is good only on the machine you install it on.
As soon as you replace your motherboard it is deemed another device and the license is no longer any good - as OEM licenses cannot be transfered.
So work out the cost of OEM and Retail.
If Retail is twice as much and you plan on two or more "upgrades" or machine swaps during the life of Vista then retail works out best.
If you plan on building a PC, not replacing the motherboard then OEM would be the cheapest option.