I'm going to have to disagree with you in a huge way.
445Mhz being insanely low equates to 1780Mhz.....

Just in the last year or so have boards been spec'd at 1066Mhz / 1333Mhz.
For a long time, anything over 400Mhz FSB was considered to be the ultimate clocker, so I'm surprised to see you write that the lowest you've ever seen is 460Mhz???

Anyways, aside from that, I'm not exactly sure how to go about finding out if the wall is the CPU or the board itself....outside of putting another CPU in the system and seeing how high it clocks.
I know it's not the RAM.
I can run unlinked with this board, and the RAM itself has been up to 1282Mhz.

With the E6420, I have 3 options: 6x, 7x, or 8x multi.
445'ish is the limit, regardless of multi, so I can't determine from that whether the motherboard or the CPU is holding back the higher speed.