Did I make a bad decision on GPU ?

To make this short, I just ordered a radeon hd 4890 for 195 bucks on the egg. But then I started banging my head on the wall because all along my motherboard supported crossfire which I knew but never really cared about it, So I didn;t think it through much and just bought the 4890, then I noticed that on ebay brand new 4850's were up for 95 bucks. 95 + 95 = 190. I could have had a 4850 crossfire instead of 1 4890. Im assuming that 2 4850s will outperform 1 4890 or am I wrong ? Dont even mention getting another 4890 because im not paying another 195 for one lol. BUT I WILL get a 4850 and crossfire it wiwth the 4890 but im afraid it'll just bog down performance instead of helping it, or am I wrong here too ? Can you guys help me out here im pretty much a noob, How would a 4890/4850 crossfire do ?

1920x1080 and yes i can handle 2 cards, 700w ocz psu.

Two HD4850s would've been better I suppose but an HD4890 should be pretty nice for that resolution anyway. I would just try it and consider xfire only if you are unhappy with the performance or perhaps get one later on when another card would be cheaper and games more demanding.
I guess you could always ebay the HD4890. You can probably get as much as what you paid for it or close.