Not sure if it would need less cycles as either MPlayer or the GFX-card (if overlay is used) would still need to do some scaling.

It will need less VRAM as such low res would really be an oldskool 31kHz (aka VGA) signal send to the monitor (regardless wether you use VGA or something digital like DVI or HDMI).

Real performance issue will only arise when you have to many screens and windows open and run out of VRAM. This really only an issue for systems with 64MB or less,