Might help if you gave us a little more info -- was it a window unit or an outside-unit (central air)? Size of the house? How's the insulation? How 'bout the air-handler unit (if you have central air) - is it sized right?
I just had my outside unit replaced this week. Got an heat-pump to help (maybe) reduce some of the heating costs this winter. The price of propane has gone through the roof, so wanted to try something different. And if you don't have enough air-returns, it won't heat/cool the entire house properly, either.
Advice - have a pro come and size it. Have several come in and get estimates. If it's too small, it'll work too hard and burn itself up. (That's what happened to our old unit...) I think they also said that if it's too large, it won't defrost right, which will cause other problems.
Higher SEER units are more efficient, but more expensive. You'll probably want to go with a 12-14 SEER -- that seems to be about the best balance (efficiency -v- cost).
Estimates I received were from $1600 - $3600 in the 4-5 Ton 10-14 SEER range. Higher price was higher capacity, higher SEER.