I have a 5 yr old Dell Inspiron with core2 Duo processor and 4Gb RAM running Win7Pro. This is not a fast machine by modern standards.
USB 2.0 is fast enough for transferring and it manages D800 files OK but is definitely slower than the D300 files. I am using a SD card as primary because the laptop has a SD card reader built-in and it is a bit quicker than the USB 2.0 port.
It really battles to run LR4.1 and I get frequent crashes and long processing times. Nikon Capture NX2 runs much faster and luckily, I prefer it over LR anyway. I try to process D800 images individually because it struggles if I try to load up 5 or 10 NEFs at a time like I could with the D300.
My reason for sticking with the ageing Dell is its great 1920x1200 17" display (16:10 aspect ratio) which is no longer obtainable in the modern 1 age.
I am eyeing out a new machine though - trying to weigh the difference between the advantage of faster processing or the joys of a new lens. As an enthusiast my current system gets the job done in the evenings after work but if I was a pro trying to handle 10 to 50 times the volume of images I would definitely want a much faster machine.
If you have the glass, D800 images are so good that I would not let a slow computer hold me back from buying one. If you can afford a D800 and one or two nice lenses then you can afford a new laptop in a few months time