By the way, I do not have an iMac with a Radeon videocard.
I have got the 24" iMac with the NVIDIA GeForce 9400M videocard.

uh,
i have the same problem.
my rom says IM51.0090.B03 but, i have a 17inch.
it is supposed to be IM52....

my imac keeps crashing.

Apple is sometimes lazy about properly identifying which updates apply to which Mac models. Your Macs are "Late 2006" models, and even though the display size is different, the logic board, etc. are similar enough so that this firmware update applies to both. If the firmware update wasn't appropriate for your Mac model, its installer wouldn't have installed it. Any problems that occur after any firmware update aren't due to a mismatch between the firmware update and a given Mac model, but due to bugs in either the firmware update itself (which means both some 17" and 20" models may experience similar problems), or your System software, leftover cache files (clear them out using something like Leopard Cache Cleaner), etc.

steelcut and ambient:

I just doublechecked your Mac's firmware numbers, and you're right, the model ID shown in the firmware version that the updater has installed on your Macs, doesn't match your Mac's official model name (17 Inch Late 2006 CD)--IM51 is the model ID for the 17" and 20" with a Superdrive, and IM52 is what you'd normally expect with a 17" CD model. Also, IM51.0090.B03 isn't one of the numbers Apple shows at http://support.apple.com/kb/HT1237, even for non-CD models--it shows IM51.0090.B09 as being what firmware update 1.2 installs for both the 17" and 20" Late 2006 models (which is a further oddity--if you installed firmware update 1.3 and got firmware IM51.0090.B03, it seems that the 1.3 update might install a variation of the 1.2 update on models that don't support the 1.3 update). But since the 17" Late 2006 CD model is just a 17" Late 2006 model with a Combo drive instead of a Superdrive, their logic boards are probably identical, or close enough (maybe the only difference was the model ID originally in their firmware), or some production runs may have used these two logic boards interchangeably. So, it's likely that these inconsistencies aren't anything to worry about. Still, it's debatable whether it was a good idea or a bad one, for the updater to change your Mac's model ID to a technically different model's. It's even possible that your 17" CD model's firmware originally showed IM51 as its model ID, if it contained a non-CD model logic board all along.