While iit doesn't always cause the activation check if there were others then it can happen. Microsoft tells more...

"How does product activation determine tolerance? In other words, how many components of the PC must change before I am required to reactivate?

Common changes to hardware such as upgrading a video card, adding a second hard disk drive, adding RAM or upgrading a CD-ROM device will not require the system to be reactivated.

Specifically, product activation determines tolerance through a voting mechanism. There are 10 hardware characteristics used in creating the hardware hash. Each characteristic is worth one vote, except the network card which is worth three votes. When thinking of tolerance, it's easiest to think about what has not changed instead of what has changed. When the current hardware hash is compared to the original hardware hash, there must be 7 or more matching points for the two hardware hashes to be considered in tolerance. If the network card is the same, then only 4 additional characteristics must match (because the network card is worth 3, for a total of 7). If the network card is not the same, then a total of 7 characteristics other than the network card must be the same. If the device is a laptop (specifically a dockable device), additional tolerance is allotted and there need be only 4 or more matching points. Therefore, if the device is dockable and the network card is the same, only one other characteristic must be the same for a total vote of 4. If the device is dockable and the network card is not the same, then a total of 4 characteristics other than the network card must be the same.
Are the changes cumulative? In other words, if I change one component today and one tomorrow, is that two component changes?

The changes are cumulative; however, if a user is asked to reactivate, the hardware profile is reset to that new configuration.
What are the 10 hardware characteristics used to determine the hardware hash?

The 10 hardware characteristics used to determine the hardware hash are: Display Adapter, SCSI Adapter, IDE Adapter, Network Adapter MAC Address, RAM Amount Range (i.e. 0-64mb, 64-128mb, etc), Processor Type, Processor Serial Number, Hard Drive Device, Hard Drive Volume Serial Number, CD-ROM/CD-RW/DVD-ROM."

Read the full version at http://www.microsoft.com/piracy/activation_faq.mspx

Bob