I suppose I need to take a look at a lot more reviews to be able to make any claims.
http://www.guru3d.com/article/geforce-gt-220-review-test/3
http://www.guru3d.com/article/radeon-hd-5570-review/6
http://www.guru3d.com/article/msi-geforce-gt-240-review-test/5
http://www.pcgameshardware.com/aid,697132/Geforce-GT-220-reviewed-Nvidias-DX101-graphics-card/Reviews/?page=12
http://www.xbitlabs.com/articles/video/display/radeon-hd5670-hd5570-hd5450_6.html
In contrast to the others, this one favors the 220/240, but just barely.
http://www.legionhardware.com/articles_pages/ati_radeon_hd_5570,8.html

I think there's a pretty big difference between "minimum" and "recommended", too. ATi never states a minimum, just a recommended.

I don't know anything about those nVidia cards, but if the lower Radeon HD 5xxx cards do the same thing the higher ones do, they will use little power when idle. The higher ones automatically underclock to something like 100 or 150 MHz, IIRC, and maybe lower the GPU voltage, too.

This page shows a pretty good list of video cards and how they compare, the GeForce GT 220 is a decent card for the price, and it should be able to run SC2 without a hitch, and it should last you for the next couple years, I'm running on an 8600 GT and it's worked fine for some of the higher end games (Assassin's Creed, Sims 3, Sacred 2), and the GT 220 is even better.