How high can the Opteron 175 OC to?

What kind of cooling are you using?
It has been my experience that the Amd chips OC better over time. You will be able to OC higher in 2 months than you can next week.
For the best info on OCing, check in the OC forum.
Star simple. Reduce your HTT multiplier to 4, and increase the base rate from 200, to 220. Run it that way for a month.
If you are using DDR400, you may have to lower your ram to 333. (the 220 base will raise it to 366, while @ 400 rate, it would have to be able to handle DDR440 speed.)

I agree in general, although 220 for a month is a bit conservative. You're not getting off the 11x multiplier.

Lower HT to 4x, ram to 333, and get a good hsf, zalman is my brand of choice, but there are other good ones. you should be able to run 230 for a while with that, if it runs well, have fun trying to hit >2.7 [dont kill the chip though].

How long have you ben running your 165?
When it hits 2 months, I bet you will be able to go higher.

This may be of some interest to you

http://www.hardwareanalysis.com/content/topic/53920/

Somewhere between 2.7 & 3- I would be careful of the long term effects if OC at high end.

I am using Thermaltake Silent Boost