Help me decide

I have a Tempstar 5000 (4 ton) 10 SEER  air conditioning system. It is 11 years old. works fine I never had a problem with it. Never done any tune ups or maintance to the system.

I am thinking of replacing this with an Energy Star system that qualifies for the $1500 tax rebate.
I am looking at 16 SEER systems and I was told that I have also to replace the furnace in addition to the air conditioner and coil.
Any thoughts on this or recomendations? what systems should I consider? Any ball park figures?

Thanks,
Daoud