Is upgrading an iMac to an SSD over USB 2/FireWire an upgrade?
There would be a major speed bump. Not as fast as having the drive on an internal SATA interface but noticably fast all the same.

I ran a four and a half year old iMac 24 inch on a Crucial M4 SSD via a Firewire 800 enclosure for about 5 months to get a major boost over the internal SATA HD.

You'll notice an immediate improvement to random read/write speeds, day-to-day things like app launch and system boot times. Sustained rates will only be as fast as the interface you pick and may not be any better than your internal drive. However since most OS and app disk activity is random this isn't too important.

With Thunderbolt available on your system a future SATA 6Gbps enclosure to Thunderbolt interface will remove the sustained transfer rate  limitation.

Avoid USB 2 as you will never reach the theoretical 60 MB/s due to USB protocol overheads. I have only ever seen a maximum of 35-40 MB/s over USB 2 hence my choice of a Firewire 800 enclosure.