Something to remember:
As expensive as gas is, it takes a lot of driving for these mods to pay for themselves. For example, say that adding a DSII gave you 1 mpg over the load-o, moving you from 16 to 17 (arbitrary).
Also assume you commute 100 miles a week to work in the car. (More like a 100 miles a day for me, oh California)
That means that, In 1 week - you used 100/16 gallons with the old setup, or 6.25 gallons. With the new, thats 100/17 or 5.88 gallons. @ 3.50 a gallon, that means you save 1.29$ worth of gas per week.
So, if you spend 129$ upgrading to DSII (approximate and keeps math easy) it will take you 100 weeks, or two years, for it to pay for itself.