The fuel savings between a Prius and a 20mpg car in the US works out to about $9000 over 150,000 miles. I'm not sure what to put in for a car's "lifetime". Mine have 160,000 - 380,000 miles on them, currently. And range from 24 to 45 years old. I don't think that's typical. The one with the 380,000 miles on it is my 1988 Chevy Sprint Metro. (Really a rebadged Suzuki.) It beats the Prius, with an original 55mpg city/60mpg highway EPA rating. The 2008 adjusted numbers being 44mpg city/51mpg highway. (In which case it only beats the Prius on the highway.)
The Sprint has saved me over $30,000 in fuel cost over its lifetime, compared to the 20mpg car you suggest for comparison. Not including the avoided cost of buying new cars to replace it. (Suzuki reliability was amazing.)
The "advantage" to throwing away old computers and replacing them with new, more fuel efficient ones has always seemed a bit iffy to me. I support the practice. But I'm not sure it makes economic sence based solely upon electricity savings.
On an absolute scale, looking at fossil fuel usage kilogram for kilogram, more efficient cars are clearly more important than more efficient computers.