The problem is the current entry-level/mid-range NVIDIA GPUs aren't as good as AMD ones.
You can see this by looking at the countless video comparissons on youtube between the graphics in video games, personally I was looking at the AMD R9 390 8GB which was gonna cost me 370 €. For that money, I can get a NVIDIA GTX 970 4GB and the problems start right there, half the VRAM for the same price, and when I see FPS comparissons between both cards in regular AAA games, the R9 390 wins more often.
So the R9 390 would be a smarter choice, specially since it's more future proof with the 8GB VRAM but I know where you are coming from Jean, everyone running SL in and AMD R series is complaining about bugs, artifacts and glitches and the only way to get rid of those is to lower the graphic settings, not something you'd wanna do on a piece of software that came out in 2003, I know I know, it's a million miles from what it was, but who can resist?
Personally I don't wanna get the GTX 970, because the problem with it's 3,5+0,5GB VRAM (google it) honestly what I'll likely do, and recomend to anyone thinking about buying a computer right now for SL, is to wait till spring (2016) when the new line up of GPUs is probably gonna come out, here's hoping NVIDIA will do a better job, because I'm not one bit confident AMD can suddenly fix all the problems it always had.