nVidia has put itself once again at the forefront of PC technology and despite AMD and ATI now merging, nVidia has no plans to stop making chipsets for this platform - in fact, they want to beat AMD/ATI at their own game.
nForce 500 series is pretty well evenly matched, at stock speeds there is little to no difference between the four variations. When going SLI, the two x16 slots vs. the two x8 slots don't really make that much difference. 590SLI is really only going to be useful for the GeForce 7950GX2 users who must have twin x16 slots to run Quad SLI but for the standard SLI users, 570SLI fits the bill just fine.
With the 590 and 570 series both supporting the same amount of built in chipset features, if you aren't going to use twin 7950GX2 graphics cards in Quad SLI, it's hard to recommend anything but the 570SLI as you're saving money choosing this platform.
For the value end, the 550 fits the bill for Sempron users no doubt. And if you are after a bit more grunt, 570 Ultra makes a good companion as long as SLI is not a requirement.
For all our testings we can't go past the value and performance of the 570SLI - when it comes to Socket AM2, it is by far nVidia's best value/performance option. On a side note, we were impressed with our initial results of the DFI NF590 SLI-M2R/G Lanparty motherboard and we'll be taking a closer look at it soon.