Hi, I've been playing the game and first let me congratulate you for the idea, and the linux port, I'm loving it :D.
My computer is:
Intel i5, 4 GB of RAM, 1GB Radeon HD6550M. I'm running Arch linux 64bits with the latest proprietary driver.
That being said I'm having some problems with the game. First of all sometimes the game is really lagged, and then it runs in fast speed to make out for it. I can play for some time before experiencing this problem, or it can happen about 20 seconds appart, seems kind of random.
Also sometimes in the menus there is this strange polygon glitch, in which lots of multicolored triangles are being shown instead of the menu itself (it seems really weird, kind of like drawing a triangles in openGL and giving them different colors to each of its vertex insetead of a texture).
I'm guessing that both of this problems have something to do with steam recognizing my video card as having only 256MB of RAM. and therefore the game takes a while to load the textures and doesn't keep them in the GPU because of a size limit that doesen't exist.
I've experienced both of this problems also on Brutal Legend, however there it doesen't happen this much (about 99% less than in stacking, exept sometimes with the car where the game gets just as lagged)