You will be fine regarding power of your desktop unless you are using a laptop. Even though the files are larger, processing time isnt that much longer - theyre not 300mb TIFF files so its alright. You honestly dont need serious PC or Mac power - Ive got a 3 yr old comp here, 2 gigs of ram, duo core, and some older components. The difference between my 1DsMKII files and 5DMKII file processing times on Lightroom is not that much noticeable between the 2. Unless I open them up in ACR for CS4 then it drags on a bit longer, due to it being a ram hungry hog.

Re. ISO you will notice a big diff at 1600 between the MKII and classic. The MKII high ISO is more smoother and has less chroma noise also. Another point is that the classic is sharper right out of the camera - due to having a weak AA filter, that and the original 1Ds produces some of the sharpest photos with the right lenses for Canon due to having the weakest AA filters.