One GByte of RAM is a bit minimum for any of todays 3D applications. I imagine a lot of off-loading to the hard drive is driving up your render times?

I would go for at least 2 GByte of ram, with 3 pretty much all your system can use. ~ 1.8 GByte is all carrara gets -- under ideal conditions, and assuming a good stable motherboard/RAM.

If you're using C5Pro, try Ambient Occlusion Only, rather than full Indirect Lighting. Ambient Occlusion does a good job of simulating real indirect lighting. though it's probably really best for animations.