Just for reference, MicroCenter now has Win 7 Pro (32bit and 64bit) OEM copies for $129 (normally $149) until 11/22/09 (microcenter.com). I'm not sure but it may be in-store only. Just thought I'd mention that.
They also currently have other flavors of Windows 7 on sale as well (I think Home Premium 32bit and 64bit OEM are $99 or $97, and I think ultimate is around $199 for the OEM copy).
Heck, it's currently cheaper than the XP Pro or Vista Business OEM copies, so you might as well get Win7 if you plan to upgrade......