Since you apparently intend to use your external for your clone, blazing speed wouldn't appear to be critical, so you might do well by returning your iMac and getting a newer one that supports USB 3...faster than FireWire 800...slower than Thunderbolt.
In total, a newer iMac plus a USB 3 external may cost no more than what you've got plus what you're contemplating.
The new Great Equalizer is the SEND button.
In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
