Don't skimp on your SSD. 256 is enough, but 128gB would be pushing things once you've got Xcode and all your docs, manuals, books, technotes, example code etc. loaded up.
Stop buying Firewire drives for backup purposes for certain. USB 2 speeds are livable as long as you're not moving 150+ gB a day. No point connecting SSD thru USB 2, so for backup, go with USB 3 devices. They're USB 2 compatible, and when you finally do update to a USB3/Thunderbolt machine, you wont be left with a pile of slow storage.
Incidentally all my Firewire drives turned out to be SATA, so I could pull the HD out, and stick it in a USB 3 caddy for a massive speed improvement.