I can't speak to a 2008 iMac, but I have a 2011 Mini and a 2013 Macbook Pro Retina. The only way I could get 8.0, 8.1 and Windows 10 Preview on both of these was to:
a.) download the whole Windows 7 USB stick (one for each - it doesn't 'travel') that Bootcamp Assistant demands of us (Apple's up-to-date Bootcamp software download combined with your own internet downloaded Windows 7 .iso image that I no longer know how to find online - I saved one a year ago and just re-use it). You NEED the purchased MS Key # from a DVD, but the DVD itself is useless to Boot Camp.
b.) create a fresh partition using Boot Camp and allow the fresh W7 install to happen, then
c.) activate W7 online and even before you update it at all,
d.) fire up a Windows 8.0 Disk or usb stick AND INSTALL 8.0 FROM INSIDE Windows 7 side as an upgrade. It just works as soon as it's plugged in.
You can see where I'm going with this.
Windows 7 is a great base to upgrade to 8 or 10 from (in a boot camp situation). After 8.0 installs you can slog through all the updates (there are many) until it offers you 8.1 (which is better) and then you update THAT until they offer you Windows 10 (available July 29th).
Some "clean install" issues you might encounter: When you upgrade from 7 to 8.0 there is an option to 'keep' all your personal files and preferences etc. or to "keep nothing" which is a clean install. Sometimes the "keep" option goes sideways (more than once for me - it certainly takes a lot longer) and then you HAVE to re-start and go with a clean install. This will erase your old install with your Boot Camp Apple drivers. The remedy then is when the upgrade to 8 or to 10 is done - re-insert your Boot Camp created W7 usb stick and find the Apple Drivers "setup" icon deep inside - you'll know it when you see it. Clicking this will reinstall the W7 Boot Camp Apple drivers and tray icon (which works fine on both W8 and W10).
Generally I find that anyone who can run Windows 7 on Boot Camp can also upgrade to 8.0, 8.1 and 10 as well. There is just NO GUARANTEE that you can safely upgrade while saving your current Windows 7 files and preferences - maybe - it's a dice-roll. I would count on losing everything. So if you can SAVE all your W7 files and favourites and preferences, the "clean install" process should work. Keep in mind it takes time - I've had some stubborn 8.0 updates that wouldn't install (there are a ton of them). After I left my Mac on for a few days without touching anything the downloads FINALLY happened - and you cannot go to 8.1 without those updates.
As far as W10 goes - I've had the Preview .iso for some time (burned to DVD) and THAT installs over 8.0 no problem, but W10 doesn't update very well, as it is a Beta. PROBLEM here: they say these W10 Preview .iso files are unavailable from now until the 29th official release date. In THAT regard, my post is useless.
Basically if the little "Windows" icon currently appears on your W7 or W8 toolbar and it is offering you the July 29th upgrade for free - remember that even though it is designed to install WITH all your files intact, it's predecessors are far from perfect in that regard. And also that there is no safely "going back" if you don't like it, or a year from now you don't want to pay.
Don't forget that going backward is no fun and frought with danger - you are pretty much guaranteed to lose your old install no matter WHAT they promise with "Restore" procedures. Restore on Windows is crap when it comes to reverting to an older OS. You WILL be doing a clean install if you go backwards - so save your valuables.
FINALLY - I haven't tried this but there is a way to 'check your hardware' for Windows Compatibility [I feel it is unnecessary] see here -
http://windows.microsoft.com/en-us/windows-8/upgrade-assistant-download-online-f aq
