Just yesterday I was reminiscing the stability of Sun Sparc servers. I recalled the time we moved offices in 1999, and we had to shut down them down. One last peek at the uptime showed they'd been up and running for 6 years or so (turned on when we moved in, turned off when we moved out).
I seem to recall it was a SparcStation 10. No clue about the clock speedm but I believe it was more. Note: my first PC was a 386/25MHz, which was considered blazingly fast (and a cool 10k including the Nec MultiSync 3D monitor, a 256 color EGA video card and 24 bit P2200 printer).
PS A different topic altogether, but for quite a while it seemed that whatever clock speeds we gained was wasted on pushing pixels around on the screen...