Vista OEM Install and Windows 7 Upgrade

I have (2) Dell T3400 workstations that came with Vista Business OEM COA product keys, however, these PCs have been running with XP Pro SP3 using retail installs. With pending end-of-life of XP looming, it's time to migrate to Win 7, however, there is no direct path to "upgrade" from XP to Win 7, except a clean install.

My plan is to upgrade the OS of both machines to Vista Business, and from there, upgrade to Win 7 Pro. This should be doable per the version upgrade paths available for each OS. Since I am not planning a clean install of Vista, rather an upgrade, I can't use the Dell Vista restore DVD, rather will be using a retail Vista Business upgrade.

Per this plan, I have performed the Vista upgrade to the first machine, using a retail Vista Business SP1 Upgrade DVD but installed using the OEM Vista product key on the Dell computer cabinet for that PC. This seems to have worked OK (after applying the various new drivers and other software updates). Windows activated and runs OK with this process. Note: this first computer had the original CPU and mainboard as it came from the Dell factory.

Now, here's my question: the 2nd Dell workstation, identical model to the first but still running XP, has had the cpu upgraded to a faster chip than it came with originally (from an Intel 2.66 GHz Quad Core to an Intel 3.0 GHz Quad Core Extreme). The faster chip was removed from a 3rd Dell workstation since taken out of service. In order to use the OEM Vista COA product key for this second machine, I know the OS is tied to the hardware it shipped with, however, is that product key enabled by that specific Dell mainboard & service tag, or is it tied into the cpu serial number as well?

Bottom line, in order for the second machine’s OEM Vista product key to work on that second machine, is it sufficient that I am reinstalling to the same Dell computer & service tag and original mainboard, or do I also have to downgrade the CPU chip back to the original that came with the computer for this key to work?

As it doesn't make a lot of sense to pay $100+ for a retail Vista upgrade if the OEM Product Key will work under the right conditions, I just want to understand my options before I get started to save some frustration along the way.

Thanks in advance for any thoughts.