I just put in a 7 lb Fidanza flywheel in my homely little DX and I'm pretty happy with it. Mileage has gone up just a bit and acceleration has been helped as well. The downside is that the car is a bit harder to drive now (harder to get the car moving). I only replaced it because I had to replace the clutch and I knew I wanted to replace it at some point anyway. I figured that since I was in there I should just go ahead and do them both at the same time.

As for turbos, if I were you I'd stay away from them for now and do other things to your car. Turbos are very expensive and complicated, and if you don't know what you are doing you could blow your engine very quickly.

Have you done anything to your car yet? What is your car used for? If it's your driver, you don't want to over-do it as you could make it unreliable and inefficient if taken too far.