Currently have three boxes, (one laptop and two desktops), running Vista Home Premium, (Ultimate didn't have any extra features I wanted), one of these is retail and the others are OEM. There's also another laptop and desktop running XP Pro, (one retail and one OEM) and I think I have a spare XP retail license as well where I've cut down on machines.
There's a few other machines but they are running non-Windows OSes (and are licensed too).
I am tending now to use OEM licenses as I don't tend to upgrade very much once the system is built, (at least not the motherboard anyway), and even if I do upgrade something which requires a new license then it takes several purchases of the OEM version before you get to the cost of the full retail product.