Ok, I am in the process of picking a motherboard and I am not sure which northbridge I should get.  My CPU is AMD FX-4100.  I am stuck between a 970, 990X, and 990FX northbridge.  I really don't want to spend much more than $120 on the motherboard if I can help it, but at the same time I want a decent northbridge so I won't have to upgrade in 2 years or so.  To my understanding, the only real advantage the 990X and 990FX have over the 970 is that they support SLI.   I am not really sure what the difference between the 990X and 990FX is.  http://www.newegg.com/Product/Product.aspx?Item=N82E16813128510 this is the motherboard I am currently looking at.  I am not sure if the 990FX means it goes well with the FX series of processors or if the FX is just coincidence (I kind of just noticed that right now).  So yea, any thoughts or suggestions would be greatly appreciated.  Here is my CPU http://www.newegg.com/Product/Product.aspx?Item=N82E16819103996.  I got it on a sale for $90 which is pretty good I think.   This is my first build btw and it's been a rather longer process than I had anticipated (extra spending money is hard to come by when you have bills and school).

The only difference that I found between them is the SLI/Xfire support.

www.anandtech.com

970- no.

990X- SLI and XFire x8/x8

990FX - SLI and XFire x16/x16

The rest seems to be the same for all of them (nº of USB ports, nº of SATA 6Gbps, etc).

The choice should be easy, if you plan to go with only 1 gfx card, the 970 chipset should be enough, and if you plan to go with 2 cards in SLI/XFire then the 990X is the chosen one.

Are you really confident the FX-4100 is a good gaming CPU?

According to here: http://techreport.com/r.x/desktop-trinity/trinity-99th-discrete.png

The $95 A8-3850 is faster for discrete GPU gaming, and the motherboards are cheaper for that. Or if you spend a little more you can get a much faster Core i3.