Medium Quality Performance
Being the sticklers we are, we typically look at 60fps as the ideal figure for smooth gameplay, but we think you can comfortably play SimCity at 30fps, so just about every card we tested -- including the dated HD 6750 -- should suffice if you're willing to play on medium quality at 1680x1050. That said, we'd still aim for the 40fps mark or so, for a little padding, and that still only requires something in the ballpark of an HD 5870 or the GTX 660.
Scaling up to 1920x1200 shaves at least a couple of frames off most of the cards, which inevitably makes some of the lower-tier offerings a little less desirable as they fall below 30fps, but you can still get by with an array of budget-friendly products. Hitting 40ish frames got noticeably pricier, however, as even the GTX 680 and HD 7970 hovered just above the threshold, with the latter performing 3fps better.
If you're going to play SimCity at higher resolutions such as 2560x1600, you'll definitely want at least something that falls in line with today's mid to upper mid-range performance, with the HD 7870 delivering 32fps and the GTX 660 squeezing out one extra frame. Unless you plan to buy a Titan, playing with an average of 40fps with today's software isn't likely to happen with an Nvidia card, while the HD 7950 Boost and up on AMD's side still managed to average 40fps or better.