In my opinion, the CRX HF w/ d15z1 is the ultimate conventional car as far as fuel economy goes. One person on GS'ers sporting a gutted version was getting 80mpg without any significant aeromods. With very extensive aero/mods, 100mpg should be within reach in conventional driving (i.e. no P&G).
Personally, I'd follow MetroMPG's advice and work out the budget before you buy. It all depends on your goals, but it will probably be cheaper to keep the Swift.
If you have the difference between your Swift and the Insight in the bank, I'd invest it in a CD or something similar. If not, then I would use the money you save by driving the Swift and build up a similar investment. When the Swift finally becomes uneconomical to own, you should have a nice sum of money to fully purchase the car you really want.
The more you change cars and/or stay in debt, the less you'll save. There's no sense in torturing yourself over a couple bucks, but if a new(ish) car is just a "nice to have" type thing, I'd hold the urge. Easier said then done, though...right?
- LostCause