The FX series processors were not well received in the marketplace. They are in many ways worse than prior generation technology from AMD.
They also don't have built in graphics like the A series Llano chips do.
The motherboard you chose has onboard 4250 graphics which are probably not directx11 compatible whereas the A8 chip graphics uses 6000 series chips which most likely are directx11 compatible.
That could make a difference in some applications.
Also, I don't know what sellers you are talking to, but I don't trust them to know what they are talking about whoever they are. The webpage that the motherboard maker built to show this product clearly doesn't say FX in the processor options.
http://www.gigabyte.com/products/product-page.aspx?pid=3475#ov
Supports next generation of 45nm AMD AM3 CPU
This motherboard designs with Split Power Plane, 4+1 phase VRM to support AMD the latest 45nm AM3 Phenom™II/ Athlon™II processors, delivering the great performance enchantment to system and ultimate scalability hardware expending.
I see Phenom 2 and Athlon 2 written in there, not FX.
It is possible that a motherboard update may make it work, but if so you will want to make sure the seller does it before they give you the PC, because you won't be able to update the BIOS yourself unless you have a processor that works laying around in addition to the FX-6100.
If you have to buy 2 anyway just to update the BIOS, you might as well skip the FX and just stick with the better older one anyway.
If you just do as I mentioned, you can avoid buying a video card at a later date for the most part unless you become a hardcore gamer and decide you must have the best graphics performance.
Even if that situation did occur, you likely wouldn't want to have the FX-6100 installed for that anyway.
If you do want to add a video card at a later date you should plan for that now with the PSU purchase. The Alpine 600 won't work with most mid range and above video cards. If you want better than the A8 llano will give you then the graphics will probably use more juice than the Alpine will put out.
The PSU wattage actually makes very little difference in the overall functionality of the PSU. The Antec EarthWatts 380 that I suggested is capable of delivering more power than the Alpine 600w is. The devil is in the details. Not only does it put out more power it also puts out safer power too.
Neither one will work if you want to add a high end video card at a later date, though.
I would get an XFX 550w if you want the possibility of adding a mid range or better video card in the future. This can handle pretty much any video card you would want to throw at it now or in the future and it is a very well engineered PSU in general.
If you would like to get into further details, my ram and DVD selections both have much lower failure rates as well.