The math is very simple, the better is the one with greater performance/price ratio.
Because both CPUs have the same architecture and the same features and caches, the performance scalles linear to the freqfency. So, you can calculate the factors by dividing the freqfency with the price.
Because we don't know how much each will cost you, we can't make an exact conclusion. But, acording to the www.newegg.com prices
http://www.newegg.com/Product/Product.asp?Item=N82E16819103586R
http://www.newegg.com/Product/Product.asp?Item=N82E16819103588
Opteron 165 costs 176$, while the Opteron 175 costs 201$. The Opteron 165 has better value acording to the peformance/price factor
If you plan to overclock, take into account that the Opteron 170 has more multiplier options to choose(11 compared to 9 of Opteron 165), but it will OC less in % than Opteron 165 on a decend mainboard.
I think that you are going to make a wrong choice going after Opteron. For 220$ you can have an Athlon64 FX-55 2.6GHz and a 250GB hard disk.
http://www.newegg.com/Product/Product.asp?Item=N82E16819103527
Only the CPU has better performance/price value than both models of Opteron(165/170), it has fully unlocked multiplier(31 multiplier choices) and you'll get an extra hard disk.
If I were you, for $198( less than the sum of Opteron 170) I would get an Athlon64 X2 4200+ 2.2GHz. It is faster for singlethreaded apps, and much faster(twice as fast maximum) for multithreaded apps.
http://www.newegg.com/Product/Product.asp?Item=N82E16819103547