Uploading videos to youtube will possibly use 1% of your CPU time .. of one core ... if you round the figure up to the nearest percentage ....
The thread-handling for the current AMD bulldozer chips is not correct in windows 7 .. but apparently they will be in windows 8, giving a large boost in performance (apparently).
Current Intel ix-2 & soon to be released ix-3- series CPU's have hardware H264 video encoding, but only when using the intel video GPU .. might wait for the i3 series to launch ... because i2 series requires you to be using the onboard intel GPU, or it's processed (slowly) by the CPU.
File copying is extremely low CPU bandwidth. With Direct Memory Access (DMA) the cpu is literally disconnected from the memory bus for most of the transfer time, and is processing other things.
Transferring scanned images from USB is also very slow ... but once it actually gets into memory, a fast multi-core processor will help process images - in terms of applying different image rendering techniques/masks/correction algorithmns ... if it's just scanning and saving, a dual core CPU is more than adequate.
According to here performance was more than adequate on a phenom 2 when mixing 3 HD video streams .. a bit jerky when mixing 4 video streams, and almost unusable when mixing 5 HD video streams.
I'm not entirely sure why you need to upgrade your rig from a Intel 980X extreme for some basic video encoding ... unless you intend to be gaming & processing/encoding Hidef Videos simultaneously.
I'd have suggested that you could also use your video card to encode video's, but nvidia has been proven to have intolerably poor video output quality when using GPU encoding ...
For what you need, it seems like you could get away with a small cheap i3-2xx PC and a $50 KVM switch.