He's never gonna get it that high with DDR-333, already the FSB is stressing to move along with that much. Still, would be interesting to see an Athlon system running stably at 12.5x166.67. Thats 2083Mhz with an improved FSB!
As for an answer to his question-
Well, you can keep going as high as you can until the system gets unstable, like when you experience numerous crashes. Otherwise, leave it at 1.75, the Athlon was designed to not run at full voltage. Fifty five celsius isn't that bad, I know some friends computers who are running at 60 and 65 respectfully, albeit I wouldn't want to run at that high for too long. You should be able to get pretty high with a decent cooler. Don't expect great results with the retail OEM cooler, it won't cut it for overclocking a lot.
"When there's a will, there's a way."