There has always been a way to get x64 to install - where there is will there is way. I have 2006 Mac with 64-bit processor (Xeon) but EFI is 32-bit, which would also mean that even if I used 32-bit Windows I would only be able to use a meager 1.9GB RAM! to say I ain't about to and amused at this is an understatement.
Some require using your Windows in a VM first.
VirtualBox is free VM
Windows 8 Preview is also free (32/64 versions)
The iMac was really strange. Even the 2008s had Penryn 64-bit processor and Apple had started supporting both MacBook and MacBook Pro 64-bit (they later degraded MacBook to not being supported in late 2008 - really ).
You want Office, and it might run just fine in 32-bit in a VM so another reason to try VirtualBox - I know Excel runs better natively
Macs that work with 64-bit editions of Microsoft Windows Vista and Windows 7
If you see this:
"Installation error: Boot Camp x64 is unsupported on this computer model."
Means just use control+click and "Troubleshooting Compatibility Mode" and/or use the BOOTCAMP64.msi app directly instead of SETUP or anything else. I know Lion Boot Camp support software is even more agressive at this whole "unsupported" stuff - very odd to say the least.