You can have both cards installed but enable/disable Crossfire from the Catalyst Control Center. If you've already got the two cards, try it out single card and in Crossfire mode and see which way you prefer. For games that properly support Crossfire, I usually prefer it enabled, even with microstutter. If you find a game that doesn't work well with Crossfire enabled, you can now use Application Profiles in the Catalyst Control Center to disable Crossfire when the game launches, instead of having to remember to do it manually. I think you have to be running the 12.1 drivers or later to get support for Application Profiles.
I'm looking for an upgrade in the next year too, but I think it's too soon to tell if the 7900 series will be a winner or not. It looks good to me, but I'm still waiting on prices to drop and drivers to mature before taking the plunge.
You'll love Eyefinity once you get it running well for your games. For The Old Republic, be sure to check out the
detailed report. It sounds like it works pretty well by default, but you may need to use the Widescreen Fixer to get a centered HUD. Good luck!