NVIDIA GeForce GT 640M - good for HD video editing?

I'm looking at buying the 21.5 inch iMac with a 2.7Ghz processor, 16GB RAM, and the Fusion Drive. I'm going to be doing a lot of HD video editing and motion graphics work on this computer, and I'm wondering if the NVIDIA GeForce GT 640M is going to meet my needs? Or should I make the jump to the 2.9Ghz with the 650M graphics card?