Well, a good temperature is ~30-35C in my book.
You can get colder, but it's only really going to help if you're overclocking, and only slightly at that. Most chips have a point where no matter what your temp is, they have hit there limit. Be it 30C, or -100C...they just won't go any faster.
You can safely run your PC up to 50C for most processors. Past that point, I'd say look into better cooling. In the 40C range is what most people run, and it's jsut fine.