Industry people KNOW that people are looking for furnaces right now. Naturally they are going to make a buck more than they would have last year before the credit. Still I would shop and get recommendations...since saving $50 isn't worth using someone who won't be around to service the unit in a year or two.
You will get far better heating bills with the new furnace as they are now using multiple stage furnaces, and blowers that suck out ever last whisper of heat. So no matter what you buy (assuming the credit will apply to the lower efficency furnaces and maybe they don't...I don't know) you should experience a significant improvement on that front.
And unlike Cat Man, I have just the opposite opinion in that the old 85% units that hooked to the chimney without the blowers and pipes sucking and blowing and condensating and which can all go bad and require service, are not set it anf forget it type furnaces.
The old 85% unit cost you a bit more in gas, but then they didn't require tuning, and adjusting and cleaning like the newer units do, and where is the payback if you have to pay out $200 per year in service calls (only slightly exaggerating here). By all means get rid of the old unit because it will cost you the tax credit and more in gas used over the next few years...so it isn't "should I" but more in line with "which one...and how much to pay"<font color="ff0000">•</font><font color="ff0000">•</font><font color="ff0000">•</font><font color="ff0000">•</font>