Depends on the bitrate you want to decode and how good the codec can work multithreaded. mplayer default only uses 1 core, you would need the mt git branch. In general you should not buy cpus with less than 3 ghz dual (2.93 is also ok). A fast dual is certainly better than a lower clocked quad for simpler codecs.
A Phenom II can play BluRay rips without any problems, even on a single core. It runs into trouble with some really heavy stuff (like the infamous birds sequence). If you get a multi-core mplayer, most modern dual-core processors will be fine.
I have two old amd processor so i can see only 720p video (while 1080p,even local on hdd aren't perfect), when i play some video (i'm using debian squeeze 64 bit) and I take a look at top I don't see difference in the use of the processors between mplayer-mt and mplayer.
I have something as:
85 % first core
25 % second core
Is it normal?
No, it isn't. Usually there should be a core which is slightly more used than the others (the one that also runs Xorg, for example) and the other more ore less similar. So, in your case, 55% and 45% would seem reasonable. Have you tried using ffplay directly? Just a thought.