BrandsImaging, we work with very large files all day every day. Right now I have 4 files for wall murals open, all 100ppi at full size, the smallest wall is 9'x21', the largest is 12'x93'. No issues here on my Windows 7 machine whatsoever.
Got a file a few weeks ago for a window perf job, PSD file, full-size at about 30'x45', built at 300ppi and with a good solid 200 layers in it. I had to move things and adjust things for the layout of the windows, no problem.
Never had a problem opening, manipulating, or saving files for busses, RVs or 53' trailers.
What people are saying, is if you have the hardware in place, either OS is good and is up to the task. If you have okay hardware and an outdated OS, don't even try. IMO, Windows 7 64 bit is up to the task of anything you throw at it, given you have the proper hardware to run it. If you think you're going to go put together a $1000 PC and have it go head-to-head with an $8k Mac, don't. But you can build a PC that will absolutely go head to head with that $8k Mac no problem. Hardware is hardware, apples are apples. None of us here are working with massive 3D animation or video files, it's all raster or vector art, any decent well-built and well optimized machine is going to handle those files with ease, regardless of the platform.
You already have PCs, PC knowledge, and PC software. Stick with PCs, save the money.