I too have vista ultimate 64, but YES you will, need to use the win7 ultimate to inplace upgrade. (at the time the RTM was released i researched it and found out that i would need to go with ultimate was pissed beCause microsoft had a cheap upgrade offer at the time. In any case go to the WIN7 microsoft web site to look at your options, may they have changed.).... I didn't i when with a clean install of win7 pro (cause i got a student priced upgrade verson).
BTW the way you can install the upgrade version on a clean computer by installinf it without the serial number, then upgrading it again, and input the serial number to activate it (while it may be 'a off the page' solution it works) and is legaller than pirating a copy.
To sum up, you are burned ( just like i was you vista ultimate), in place upgrade will require the ultimate version.
Of course this is all moot, ( except for the clean-with-an-upgrade-version method) if you want to go 64bit and are on 32 bit. (cannot do an in place 32 to 64bit upgrade)