Well in that case a 5D will be of little advantage to ya.
Plentiful Nikon DSLR's that will give you good video capabilities .. plus the added bonus of a new DSLR!
D600 comes to mind. While this body has some restrictions compared to the D800 in terms of video usage, the actual quality of the video will be on par.
Those restrictions will most likely be of no real use to you anyhow(as this stage) .. but you may find them useful as you develop your video technique(if at all).
The main feature I'm referring too is the power aperture feature on the D800 compared to this feature not available on the D600.
D800 is 'relatively cheap' nowadays, but you may need to get in soon before the Aussie dollar drops too far and makes it a lot more expensive than it is now.
Aussie dollar has dropped over 10% in the past month or so .. so be mindful.
JVC have recently released this JY-HMQ30 which has native Nikon F-mount so your lenses become dual purpose .. it is pretty new, so will be expensive(but you'll be looking at nearly $20K in Japan!)
I'd be inclined to stick with a native Nikon lens camera system, and if you do, the ability to shoot with a shallower DOF could be handy with an Fx format Nikon.
Only problem with going with a Nikon video camera is the inability to use your OM lenses at anything but close range.
IIRC, those GH2s aren't all that cheap, and by the time you add in a quality lens mount adapter, you'll be looking at not being far off the price of a D600.
Swings and roundabouts I guess .. you'll need to decide what's important to you in the long run.
(if it were me, personally, I'd be inclined to go with the Nikon camera and look for cheap-ish compatible lenses beyond that ... and .... funnily, that's actually what I did .. got the D800 tho).
As JM said, rigs will be a personal preference thing .. and very dependent on what your video style is going to be like.
Anyhow, just some more info to mull over .. probably made the decision harder now, rather than easier!