If the car is primarily a street car - meaning it's driven around all the time on local roads, highways, etc... Then the rule to stay with is what Chryco said... Keep it under your highway cruise rpm. I don't like to have any more than 2500 for most street cars. If your car is doggy with that, chances are there's something else mismatched.
If the car is dual use - racing too - then things are a little different because the assumption is you are willing to live with les mileage and more rpm, and less part throttle response that a higher flash stall will deliver - all for the sake of a higher stall speed at full throttle. For the most part, the modern higher quality convertors can feel tighter at part throttle than at full throttle. But you'll have to buy a good one for that. For the most part anything of an older design (say it's model has been out for longer than 4-5 years) will feel "spongy" at part throttle with milder gearing and higher flash stall speed.
