basically what that means is that the mobo supports RAM at 1333 standard,but will support up to 2000 overclocked(o.c.).you simply overclock it by going into your bios and then flashing your board.if you bought the 6x2GB RAM,then you would be using all the slots on that mobo.if i remember correctly,that board has 6 buses/slots for RAM......correct?you could even get away with 3x4GB RAM or 3x2GB of RAM.only thing is that you will be paying an arm and a leg for just one stick of 4GB RAM,much less buying a triple channel kit like that.look at my post on my build and you'll see how much you'll be looking at,lol.i think it's overkill personally,but it's always up to what you are wanting for yourself in the end,lol.your best bet is going to be going with the 6x2GB RAM over almost anything else.just try and keep your latency timings down around the 6-8 range for the best performance.your processor is perfect for cad.it is a quad-core with hyperthreading which will come in handy when doing cad work and having other applications running as well.your videos card is WAY too pricey for what you can get from two ati's with much better performance,graphics,3D marks,speed,etc......and for 1/4 of the price of one of those cards.check out this video card for your install:

http://www.newegg.com/Product/Produc...82E16814129130

it has the same specs as the one had picked out,but has more stream processors and with the type of mobo you are getting,you will get x16 bus speeds on both videos cards instead of just one that will give you less in output than these puppies will crossfired.

here's what i would do for your build:

http://www.newegg.com/Product/Produc...-365-_-Product

http://www.newegg.com/Product/Produc...82E16814129130 (x2)

http://www.newegg.com/Product/Produc...82E16819115202

http://www.newegg.com/Product/Produc...82E16820227401

http://www.newegg.com/Product/Produc...-033-_-Product (x3)

http://www.newegg.com/Product/Produc...82E16817153104

total price for these parts of your build will come to $1488.91 compared to the total price of $1899.95 of your original idea t build with.this doesn't include the HD's,DVD,case,OS,and so on that you had picked out.you already save $411.04 and get better performance and graphical output which is important for doing cad.plus you get the coolers for your RAM which is good because this board runs warm to hot on its own and you surely don't want to add anymore heat to it.

i do have one last concern about the heat inside the case.with the power supply mounting where it does,there is no way for the air/dust/heat to escape except from the very tiny gap just above the power supply.the power supply i suggested doesn't have a rear fan,just top,and you don't want hot air just sitting and hanging around the case and don't forget that hot air rises naturally on its own accord.i would recommend you getting this case:

http://www.newegg.com/Product/Produc...82E16811119160

because its has plenty of space for expansion and air to flow plus has enought stock fans to cool it also.in addition,you also get to save $50 on this case compared to the other case you had picked.just a thought.look it over and tell me what you think.