I have just build my first pc with Arma 3 in mind. The components I used are shown below. I built it myself and haven't worked out if a prebuilt one would have been cheaper, but I tried to stick with quality components all the way.
http://www.amazon.co.uk/Intel-Generation-i5-3570K-3-40GHz-Technology/dp/B007RUZKK6/ref=sr_1_1?s=computers&ie=UTF8&qid=1368214187&sr=1-1&keywords=i5+3570 CPU
http://www.amazon.co.uk/gp/product/B009LI7CKI/ref=oh_details_o00_s00_i01?ie=UTF8&psc=1 SSD (paid £279.14 for SSD + CPU)
http://www.amazon.co.uk/gp/product/B0085IZRSO/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1 RAM (paid £37.99 x 2no) 16gb installed dual channel
http://www.amazon.co.uk/gp/product/B007JRGW1I/ref=oh_details_o04_s00_i00?ie=UTF8&psc=1 MB (paid 125.06)
http://www.ebuyer.com/227503-corsair-850w-ax850-modular-psu-cmpsu-850axuk Power (paid £127.59)
http://www.ebuyer.com/product/240814 CASE (paid £109.99)
http://www.scan.co.uk/products/2gb-msi-gtx-660-twin-frozr-iii-oc-28nm-pcie-30-(x16)-6008mhz-gddr5-gpu-1033mhz-boost-1098mhz-cores-9 2no GPU in SLI
http://www.scan.co.uk/products/nzxt-phantom-side-panel-black-with-window CASE side window (paid £356.44 for 2no GPU + side window, including delivery all from scan)
http://www.novatech.co.uk/products/components/cooling/watercooling/cw-9060009-ww.html CPU cooling (paid £91.99) in push pull in roof of case - not my pc but show cooling setup
http://www.ebay.co.uk/itm/251261
140 front fan + blue LED (paid about £17)
200mm additional roof fan with blue LED (paid about £18)
Tray for SSD to fit in drive tray about £5
Total £1,206.19
CPU overclocked easily to 4.4 Ghz...still to get it higher...not a priority just now
Its my first pc build and I have nicknamed in The Beast.
Having moved over from the 360 and Ghost Recon, I can whole heartedly say that the Arma alpha has already convinced me I have made a wise choice to move from console to pc.