At a guess the problem is your video cards aren't really designed for megatextures and only have enough vram for more traditional textures which are simply repeated throughout the game. The huge textures in Rage take up a lot of vram and you can't just add up vram from two or more video cards. To get higher resolution textures you'd need at least 1.5gb vram on a single card and for really significant results as much as 3gb. The newest radeon 7970 with hardware acceleration for megatextures has a 384 bit bus and 3gb vram and can even use system ram as virtual memory for storing some of the megatextures if necessary. That way it pages the hard drive less and loads the textures faster reducing texture pop-in. Here's some examples of higher textures:
http://www.anandtech.com/Gallery/Album/1461#4
The upcoming Doom 4 will be the first real test of what the id tech 5 is capable of including more graphics effects like DoF, HDR, cloth simulation, and motion blur and will be compatible with AMD's new hardware acceleration. By the time Quake is released it should be possible using the newest graphics cards to see the game in all its glory on something like a 4k OLED monitor where you won't even need AA.