Antec Sonata Plus 550 and 4870x2 compatibility

It would go a little bit inside the HDD bays from pictures, if you dont have anything there, it should work fine.

How much for that 4870X2 if I may ask?

The length of the 4870x2 is 10.5". This is just under an inch longer than your motherboard is wide.
Measure with a tape measure and see if you have space. Take an extra 1/2 inch into account for the power plugs for the card. From the PCI-e backplate towards your hard drive, measure you shall.  

The power plugs on a 4870X2 is on the side BTW.

So basically, I'll have to hack away at some drive bays, i could make that work.

the 4870x2 is for 220 btw. ^^

Not to rain your parade, but Ive seen a pair for $375 shipped.
But if that $215 gives you warranty then great deal it is..

oh my, maybe i should reconsider. ^^

Also, the 4870x2 is going to generate a lot more heat than a 4850. The Sonata is a great case, but with only the rear exhaust fan, is not a great cooling solution. I would suggest a HD57xx or HD58xx series card instead and would definitely limit any OC efforts. Good luck!!!