It just makes everything so much quicker. Also, there is no noise from an SSD.
I never knew just how loud my HDD was until I got my SSD a couple of weeks ago, my rig is so much quieter now with no HDD clunking away, it's unbelievable.
The reason? well, when you're trying to find a matching card it can end up difficult. I had problems trying to find a second Jetstream as they are quite scarce now.
An SSD can be bought any time and there's loads to chose from but I would hate to have odd cards in SLI.
Kind of in agreement with the chap above (AlienALX) However unless something crazy happens, I'm positive you'll be able to pickup another GTX 670 whenever you want.
But I'd say go with an SSD for now.
Intel 330/Samsung 840 are both decent and reliable. If you want a little more kick, Corsair Force GT/Neutron GTX. But those two are fairly expensive.