1) Legally speaking, the OEM license is tied to the original mobo. For MS, the mobo IS the machine- so new mobo, new license required.
2) Now, if the old mobo is junked, there are 2 possibilities- one, it is replaced with a new Windows 7 compatible gateway mobo. In that case, your activation will remain intact if the hard drive is not wiped and the same hard drive is plugged to the new mobo.
The other possibility is that you get a non-gateway mobo. In that case, activation will be lost when you plug in your current hard drive. But you can try activate online with the COA sticker key, failing which you can call MS and explain that your mobo died etc. and they'll activate for you.
If none of this works, you'll need to buy a new license.