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What sort of editing are you doing?
if you are doing very simple edits with out much rendering, It wont make much difference in speed. If you are doing a lot of effects titles etc. you will need a faster machine with more memory. I would get FC Express over FCP and use the money you saved to buy a better machine / or more memory for the G5. you can always upgrade later to FCP
 
Answer to the original question:

Definately iMac. By the way, you could get another 512 MB RAM stick for very cheap (if not from Apple)
 
Ditto on the iMac G5. Depending on your budget, etc., After Effects and/or Motion are great apps to use along with FCP HD.
X
 
I agree with the iMac, too. It's great to use, and it would be better than the eMac for a few reasons:

1) 4:3 CRT screen vs. the iMac's 16:9 LCD
2) G4 vs. the iMac's G5
3) 1.42 GHz vs. 1.8 GHz


If you need more RAM, you can just upgrade. My iMac has only 512 but it runs well. It's rather clear to get the iMac in this writer's decision.
 
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