I'm likewise 5'7" and carry my weight well. My scale topped out at 205 and then I started to get serious about getting into shape. I started July of 2008 and as of this week am hovering around 155. (I'm no size 6 but still feel pretty great about it.)
I know how you feel about being stuck around 170. I was stuck there for six months or more. Egm took the words right out of my mouth about the weight-lifting. I can do cardio all day long, but the scale doesn't really move. Lifting weights, however, has gotten me to push past that I haven't looked back. I also really like to workout first thing in the morning on an empty stomach. Something like 20-30 minutes to jump start my metabolism for the day. If you have the time, I'd recommend it.
As for your diet, good job watching the calories. I likewise keep my intake between 1600 and 1800. Just take a look at where those calories are coming from. You could try cutting back on carbs and sugars (even fruit has sugars, so maybe cut back on fruit if you can). I've found those things to be helpful when I'm stuck.
Hope that helps! Keep posting, even if you're struggling.