Upgrade from 10.5.8 to 10.6.3 via installation DVD

Everything that I have read here and elsewhere indicates that I should be able to upgrade from 10.5.8 to 10.6.3 by installing the DVD. The iMac I am attempting to upgrade is an iMac 8,1 (circa 2008) and the installation media for 10.6.3 came with another iMac (iMac 11,3). The specs on the 8,1 iMac are as follows:

Hardware Overview:

Model Name: iMac

Model Identifier: iMac8,1

Processor Name: Intel Core 2 Duo

Processor Speed: 2.4 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 6 MB

Memory: 4 GB

Bus Speed: 1.07 GHz

Boot ROM Version: IM81.00C1.B00

SMC Version (system): 1.29f1

Serial Number (system): W88354R6ZE2

Hardware UUID: 8585BB57-C0EF-532A-BA34-AA32B2C3A6D1

However, I am consistently given the error message Snow Leopard cannot be installed on this (iMac 8,1) machine. There's 195 GB free space on a 250 GB drive, so that's not the issue either. I've got backups of all the data on the machine, but obviously, my preference is to update in place rather than to start fresh.

So, what am I missing here? Or do I need to purchase yet another copy of Snow Leopard (and risk the same things happening)?