Replacing Radeon HD 5770

I guess a 6850 would be a noticeable improvement, though not a earth shattering one.
Meeting the 200 bucks or less is kind of hard as most the performance series cards are 300+
I would probably go with this one
Newegg.com - XFX HD-685X-ZNFC Radeon HD 6850 1GB 256-bit DDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card with Eyefinity
It comes with dirt 3 as well.

The biggest difference between the two cards is the 6 series handles dx11 much better.
Though even a 5850 would be a noticeable step up from the 5770, it's just not really a gamer's card.

Actually come to think of it, you can get the 6870 for basically the same money right now so might as well spend the extra 20 bucks plus this one has a 30 dollar rebate actually making it the cheaper of the two.
http://www.newegg.com/Product/Produc...-506-_-Product

and it has more stream processors which means less choking when shaders or light maps get intense.
It's still not the enthusiast level card, but it's more than adequate to play anything on the market right now.

You'll need to measure it is longer
10.3" x 4.4" x 1.5"

What I did in my friends case was cut out one of the drive bays, since he wasn't using it anyway.
Bought another 4 inches or so for graphics card
What case do you have?

Yeah, I honestly think it's about the best you'll do for the money, you may want to wait on another opinion though as I'm a bit behind the times when it comes to nvidia options in the price range.

I'm assuming you want ATI though since you apparently have crossfire capability.

edit: even in a worst case scenario a pair of tin snips will cut out a drive bay no problem.

nah I know that thermaltake you have it's one of those where the drive bays are all one piece that covers the entire front of the case
I'm not a huge fan of that design the ones with the detachable bottom two bays seem easier to work with.