I had this same dilemma as OP few days ago. I want to upgrade but ATI doesnt support TH2Go resolutions and nvidia is taking their time to release new cards.
I hope the Nvidia stuff is good this next release. I would still be with ATI all they have to do is support a few resolutions.. It's not hard they have up to 9.8....
I can only think it's a marketing EyeFinity ploy, I hope not though.
I've been told that it's stupid not buying ATI and selling my ATI card but I'm at the point where I have no choice, The drivers are starting to get old and ATI isn't support TH2GO... I don't want to wait until the beast 4870x2 falls in price, ATM it's still holding it's value.
Going to try the 10.3 Beta release.. BUT I doubt it will work.