How good are you at OCing? Are you gonna OC the memory? Do you care about a CPU mulitpier of x10 vs. x11? I'd say get the 170, the 175 doesn't offer anymore benefit other than a x11 cpu multipier vs. the 170's x10 mulitpier. And you'll have 200mhz less to OC, no biggy.
Are you gonna run DDR400? If so, you mos definitely must get the 170, cause the 175 will do you absolutely no good. I'd say the 175 is an experts obsessive compulsive buy, cause you're really paying like $100 or more for an extra multipier option.
Oh yea, the OC to the sames speeds.