I think Mac vs. PC is more of a religious issue than a technical one!
You won't need a new PC for a higher pixel count camera, but photoshop operations and file transfers will be slower with larger files of course. As now, the faster it is the better. It all depends on how patient you are!
Both the 5D Mk II and 50D can transfer data via USB as well as HDMI. There are cards for PCs that have HDMI inputs (see
http://www.blackmagic-design.com/products/intensity/), but I don't know what sort of software you'd need to read the HDMI signal and I don't know what software Canon will be supplying with the cameras. I suspect that the HDMI output from the camera is currently intended to go to an HDMI input capable HDTV rather than a PC/Mac.
Most PC with HDMI capability have it as an output rather than an input, so they can play DVDs and Blu-Ray disks and output the signal to an HDTV monitor