On Windows Vista, I found Lightroom's user interface to be sluggish on my Athlon X2 system with 4GB of RAM. This can probably be attributed to the lower end video card that's in this system - LR doesn't use the GPU, but the OS's use of the GPU affects LR's performance anyway. When using Mac OS X on a Macbook Pro of lesser RAM and video, Lightroom's performance is better, and thus adequate.