Years ago I had an antec sonata, mostly because it was one of the first cases with 120mm fans, and designed fantastically for quiet operation, since I started upgrading ending up spending so much time taking stuff out, putting it back in I just got a case that can take a server sized mobo, plenty of clearance, mostly just so its easier to get mobo's with heatsinks already attached into and out of cases, have plenty of free space for hands to move around mobo, play with cables, etc, etc.
Get a huge case, makes messing with your computer sooo much easier, and, anything fits
How much longer is a 5870x2 over a single 5870 though? I'd imagine that will push even server cases close to its limits