*** Fibre Channel
Probably the most important thing that you'll be losing on an iMac is the Fibre Channel connection; however, Promise Technologies has their SANLink - Fibre Channel adapter using Thunderbolt Technology (I don't know if it's actually shipping yet),
Here's the product link:
http://www.promise.com/storage/raid_series.aspx?region=en-global&m=192&rsn1=40&r sn3=49
The upside here is that if this adapter works as expected, you could use any Mac with Thunderbolt (all the current models have it). Even though the Mac mini is the least powerful of the Apple line, it's a huge performace leap over a PowerMac G5 and the Mac mini supports both a computer display via its Thunderbolt/Display PortMini and a HDMI TV via its HDMI out. This takes perfect advantage of FCP Digital Cinema Desktop modes.
*** Which version of FCP do you own?
Another issue is going to be your FCP software installers (which I'm sure you're already aware of). Your PowerMac G5s must be running Final Cut Pro 6 or older as 6 was the last version to run on a PowerPC. And Final Cut Pro 5 was the first version that could be installed on an Intel based Mac.
In theory, FCP5, 6 or 7 will run on an intel Mac, however, the current Mac OS and support for it is probably going to be your biggest issue for everything that curently works as expected on your G5s.
The upgrade from FCP 5 to 6 or 7 is as easy as it gets other than having to order Final Cut Pro Studio 3 directly from Apple (last I read, they're still selling it if you call them with the part number and you say you need it for exisiting projects).
*** Do you use 3rd party capture cards like the Blackmagic-Designs Decklink? Blackmagic makes a Thunderbolt replacement which should work fine on its own, but will need to be carefully tested with the Fibre Channel adapter.
Be ready to be on the phone with tech support, checking user forums, and make sure you can take anything that you're purchased back.