I've tried them all.
Of course, originally I wanted native 1400 with at least "high" settings. I found it was doable, but just a bit jerky for my tastes.
Then I got comfortable with a few sizes down, and no "scale to panel" (e.g. I was using black bars around perimeter of play screen.) I could satisfactorily drive all settings at "high" this way and get smooth performance, especially with modest overclock (460/290).
Then I went back to native 1400, because the extra screen real estate is just so nice and fills out the field of vision. But turned everything down to medium. On this setting, the grass, trees etc. is not nearly as nice as on high, but inside lab environments there isn't as much landscape eye candy so it doesn't matter as much to me.
Stretching doesn't appeal to me much, just a personal taste. I may try it again though.
Sometimes during an outside episode I'll crank everything to high and stay in 1400, and just put up with the slight jerkiness, in order to get the kickass jungle environment.
I love my 8790 and it's a beast, but I can only imagine in my fevered dreams what this game would be like on a $5,000 AMD64 desktop. And a 19" CRT. To drive all settings to very high, with silky smooth play, is a holy grail.
Some people report getting decent play at 1400 with high or even very high, but not me. I also can't overclock more than 460/290 without eventually getting artifacts, even with good ventilation. It's fun to try though.
my machine- 8790, 3.4GHz, single 60Gb 7,200 RPM HD