I have the high end 21.5" model iMac with an upgrade to the i7 and it does video editing the level you are discussing with relative ease. If you go with the bottom line 27" you won't have an i7 but an i5 will do, especially with the stronger GPU.
I would say get the base-line 27" if you really want the screen size or do what I did and get the top-end 21.5" with an upgrade to the i7. Both cost around the same amount so you're paying for the screen size and slightly better GPU, your choice.
In either case though I would buy 8GB (2x4GB) of third party RAM and add that. It can be had for 50 bucks or less. Any of
these would do the job. The reason I say 8GB rather than just another 4 is because I did that and regret it now. If I want to max my iMac out with 16GB (technically the max is 32 but let's go with what apple says) I have to buy a full 16GB kit rather than just another 8GB one. With 8GB of RAM added onto the stock 4GB you'll have more than enough RAM for your needs and for a ration of the price apple charges.
Regardless of what you choose, enjoy the new iMac, they are great machines.
P.S though, if you don't mind waiting the new iMac's should be coming out within the next month and should have some nice additions that the current lineup is lacking. Just some food for thought though. The current ones are still more than powerful for what you want.