You don't need a 256GB SSD - I've been using a FW800 80GB Intel X25m with my 2009 i7 iMac for over a year, and it's not even half-full. I use it only for the OS and Applications, with my user folder on the internal drive, and it works great - noticeably faster and more responsive than booting from the internal drive.
You can buy a decent 64GB SSD, put it in a FW800 enclosure, and save $300-400US. You won't get the Z68 caching features (assuming Apple is going to implement them), but for OS/Apps usage, FW800 works fine.