I have just bought a 2009 4gb 24"imac 10.6.8 snow leopard running logic studio pro 8.
I was reading this thread and I should warn you that a 2408mk3 is not really the ideal I/O unit for your situation.
The 2408 is a PCIe based device, requiring a PCI-424 card to interface into a PowerMac desktop for complete 24 channel digital I/O between the D8B and the Mac. There are no PCIe slots in an iMac. You need a FireWire or USB interface to get multichannel audio in and out of your iMac. Multichannel audio to and from the D8B /iMac combo would be accomplished by either analog or ADAT/AES/EBU connections between the two.
If you're going to look at MOTU stuff for your iMac, check out the Firewire/USB line of products on their website. I personally use an UltraLite mk3 with my 24" 2009 iMac. Cubase 6, Logic 9 and StudioOne-2 all reside on it.
I also use a PCI-424/2408mk3 with my PowerMac and the D8B as my main recording setup. ( I used to include an HDR in the rig but gave it away to someone who was doing a lot of live recording and liked the convenience of the HDR on the road.) This Mac also runs either Cubase 6, Logic 9 or Studio One-2. Of the three, Cubase has the better thought out MTC/MIDI device implementation to and from the D8B, in my opinion.
The iMac now takes the place of the HDR as a sort of "mixdown machine" function. I use an AIO8 in the D8B's Alt Card slot to route either 8 stems to the iMac or alternatively a 2mix via S/PDIF from the main PowerMac/2408 machine through the D8B to the iMac.
I have found that when running this rig (and included the HDR at one time) that a master clock made sense. I use an old Lucid GENx 192 as master feeding the D8B, HDR and 2408 and UltraLite. Not click or pop anywhere.
While it works for me, an UltraLite may not be enough for what you want to do. Look at the MOTU 8Pre if more mic inputs are what you need. Or look at something like the Traveler which has 4 mic pre's and a whole slew of useful I/O including 16 channels of ADAT, line or guitar inputs, S/PDIF, WC and MIDI. It's real Swiss Army knife used with an iMac. They have a lot of solutions that may (or may not) fit your budget.
However, all that said, it sounds to me like just the D8B/HDR setup is all you really need to accomplish what you need.
If you want to further edit/EFX your primary tracks in Logic (or add virtual MIDI instruments), you can easily "Render" your HDR tracks and pass them into the iMac via Ethernet then import them into Logic. You could mix from there and cut a CD or do the reverse after editing and/or virtual instrument additions and pass it all back to the HDR via ethernet and mix in the D8B sending a 2mix back to Logic for mastering/CD processing.
It's a lot to consider. You just have to figure out what you really need to accomplish what you want to produce.