I can't tell you whether or not it's time, but I can tell you how I would think about it. I have a rule-of-thumb for how much I want a car to cost me in depreciation, fuel, and maintenance over the life of the car: about $0.15/mile. So, assuming you get 25 MPG and pay about $2.50/gallon for gas, the car is costing you about $0.10/mile. If you make the necessary repairs, the car would need to get you another 16,000 miles with no additional repairs for it to be "worth it" from my perspective (I'm assuming depreciation costs are nil at this point).
So, what condition are your tires in? Are you about to spend another $400 on rubber? What about the battery? If you think it's reasonable that you could go another 16,000 miles after the repair, then I'd pay it. (Or maybe you get better than 25 MPG, in which case, do the math yourself). Otherwise, I'd start shopping for something new. Or do the repair yourself, as zooch suggested.