Basically What's the difference between this:
Newegg.com - Microsoft Windows Vista 32-Bit Ultimate for System Builders Single Pack DVD - OEM

And this:
Newegg.com - Microsoft Windows Vista Ultimate DVD - Retail

Which do you recommend?

I happen to think I should buy the Retail, because I'd be able to reinstall in 32-bit if I don't like 64-bit with the same key...

Well id recommend the OEM copy as its cheaper, but there's a downside you can only install it and use it on the PC its intended for. If you were to build another system you would be forced to buy a new copy of Windows. The retail version however although its more expensive, allows you to install it on thew PC its intended for, and if you were to upgrade to a new PC, just un install Windows off the old one and transfer it to the new one. Hope that makes sense.

I heard that OEM copies don't allow reinstallations, even on the same system. True?

You can install an OEM copy on the same system as many times as you wish. Windows XP (as im sure you know) allows a certain amount of hardware upgrades before it needs to be re activated.