Opteron 170 is 2.0ghz & X2 4400+ is 2.2ghz ... both have a 1mb L2 cache per core.
A better comparision to the 4400+ is the Opteron 175 which is also 2.2ghz & has the same 1mb L2 cache per core... Zipzoomfly has a retail box model in stock for $513 vs $497 for the 4400+ (both prices include free Fedex 2-day) ... if you planning on overclocking I'd suggest getting the Opteron 170 though, because its almost certain to do 2.4ghz, if you want to run at default go for the 4400+.