The Fractal cases are a great, clean slate to do some mod work on.
I personally would go with the XL, but then again, I am going to put together a water cooling loop in my next build.
The NZXT H2
is another similar option to the Fractal R3 with some different features. Not sure if I'd try shoving a Multi-GPU setup in it though...
NZXT Phantom, but that has mesh on the side panels.
Aluminum Lian Li PC9F
could work with the solid side panel and looks to have enough room for Multi-GPU gear, but it's not full tower and you will likely want to paint the interior.
Honestly, there isn't a whole lot out there that meet your parameters. If you go up $30-50 there are a couple of other Lian Li options to consider and the Corsair 600T as well.
*edit* you can always check the FS Threads, there are usually some Full Tower Rocketfish aluminum cases (made by Lian Li) which are great, simple platforms to mod.Edited by whiteslashasian - 4/19/11 at 12:02pm