Can you xfire a single gpu card with a dual gpu card?

I am trying to decide on a graphics card for my new build. I have narrowed it down to the sapphire 4870 toxic edition, evga 285gtx, and a 4870x2. I'm already spending more than I planned on this build, but I want to be sure that I get enough card for my system. The monitor is a dell 24" ultrasharp, so I need a decent card.

I was reading the card reviews on newegg, and one person mentioned xfiring a 4850 with a 4850x2. Can you do that and get the performance from all three processors? I would guess the system just sees it as a three way xfire? If I can do this, I will probably just go with the 4870 for now, and wait until the 4870x2 drops in price and use that to upgrade, using the two cards together.

I am leaning towards ati right now because the motherboard I am going with supports xfire only. I am considering the 285gtx as a single card solution with the plan of selling it rather than using it when I upgrade down the line.

Which option makes the most sense?

Thanks for the link. The 285 is definately a nice card, but it looks like I will get enough performance for now with the 4870. I mean, the 4870 gets 74 fps in call of duty 4 for my monitors native resolution, which is fine. I like the idea of buying ATI since it is the underdog and it supports competition which is good for the industry.

I'm still debating it, though. I will have to add up all the other items I am buying for my build and see what it adds up to.

Thanks.

Awesome link. Thanks so much. I definitely feel better about my choice now.

Yes, when I buy my new system I will be getting a single 4870 for now. Once the price drops I will get the 4870x2.

Unfortunately I got an unexpected bill today so I will have to wait a little bit.