1:320 is high.
Some doctors will count an ANA as positive when it's as low as 1:40, while some wait until it's at least 1:160 before they'll say anything is wrong. 1:320 is the next step up from 1:160; it's usually considered pretty conclusive evidence of autoimmune activity in the body.
This article might help you understand: http://arthritis.about.com/od/diagnostic/a/ana.htm
Here's an excerpt:
Serum from the patient's blood specimen is added to microscope slides which have commercially prepared cells on the slide surface. If the patient's serum contains antinuclear antibodies (ANA), they bind to the cells (specifically the nuclei of the cells) on the slide.
A second antibody, commercially tagged with a fluorescent dye, is added to the mix of patient's serum and commercially prepared cells on the slide. The second (fluorescent) antibody attaches to the serum antibodies and cells which have bound together. When viewed under an ultraviolet microscope, antinuclear antibodies appear as fluorescent cells.
*If fluorescent cells are observed, the ANA (antinuclear antibody) test is considered positive.
*If fluorescent cells are not observed, the ANA (antinuclear antibody) test is considered negative.
A titer is determined by repeating the positive test with serial dilutions until the test yields a negative result. The last dilution which yields a positive result (flourescence) is the titer which gets reported. For example, if a titer performed for a positive ANA test is:
[In this example, the patient's ANA is 1:160 because that's the last titer at which a positive result was found]