Integrated graphics eating "into CPU resources" is a myth anyway. Nevermind it, just continue to use it. (There is a certain impact onto RAM throughput, but with current CPUs and their huge caches, it doesn't matter much either.)
Anyway, if you want a card that doesn't use much power, the size of the cooling solution indicates its power consumption best. Small, fanless heatsink -> economic card.
The true champions of making eco friendly PCIE cards are S3, coming in at 3 to 5 watts maximum power, while our popular friends ATI and NVidia clock in at up to ten times as much in IDLE. Not kidding.
Sadly, S3 have no market penetration on retail shelves at all.