If you have already applied, stop worrying! I'm just another applicant, but from what I have heard, I don't think that one low GRE score is enough to knock you completely out of the running if you have a decent GPA and good LORs/writing. Obviously it's REALLY not a plus, and you should re-take if you can/plan on re-applying in the future so that your application stands out. Some schools will be suspicious of your ability to do quantitative work. Study!! The GRE is just one of those hoops you have to jump through to get into a good program, unfortunately. If you can't re-take, though, just focus on writing a great statement of purpose and don't panic.