With $2k a build, hard not tp put in SLI'd GFX cards, "king of the hill" SSD, x16 x16 MoBo, fast CAS 7 RAM, 10.0 jonnyguru rated rated PSU, BR Reader.
EDIT ... just saw the Australia thing .... some components will be very close in price ... some will have substantial increase.
Case - $155 - Antec DF-85
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129087
Case Fans - $15 - Antec Red 120 mm
http://www.newegg.com/Product/Product.aspx?Item=N82E16835209013
PSU - $110 - Antec CP-850
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371024
MoBo - $260 - ASUS P8P67 WS Revolution
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131694
CPU - $224 - Intel Core i5-2500K
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115072
Cooler - $40 - Scythe SCMG 2100
http://www.newegg.com/Product/Product.aspx?Item=N82E16835185142
TIM - $5 - Shin Etsu
http://www.newegg.com/Product/Product.aspx?Item=N82E16835150080
RAM - $145 - (2 x 4GB) Mushkin CAS 7
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226178
GFX - $240 - Gigabyte GTX 560 Factory Overclocked
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125363&cm_re=560_900MHz-_-14-125-363-_-Product
GFX - $240 - Same
HD - $65 - Spinpoint F3 1TB 7200 rpm
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152185
SSD - $300 - OCZ Vertex 3 2.5" 120GB
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227706
DVD Writer - $90 - Plextor BR Combo Drive w/ LS
http://www.newegg.com/Product/Product.aspx?Item=N82E16827249055
OS - $100 - Win 7-64 Home Pre OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116986
$1989.....
Wanna add a very good KB and Mouse....
Keyboard - $105 - Logitech G510
http://www.newegg.com/Product/Product.aspx?Item=N82E16823126100
Mouse - $55 - Logitech G500
http://www.newegg.com/Product/Product.aspx?Item=N82E16826104318
Ya can get that total back to your $2k limit if need be by dropping $68 by skipping the BR Reader in favor of a standard DVD Burner and substituting CAS 9 RAM far CAS 7 (-$70)
Asus DRW-24B3LT - $22 -
http://www.newegg.com/Product/Product.aspx?Item=N82E16827135221
RAM - $75 - (2 x 4GB) Corsair CAS 9
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145324
That puts ya at $2011, or $1 above your original build.
The why's....
Case - Having built w/ both, I really like the HAF-X, but not as much as the DF-85....While the 5.25" lacking gates can be an annoyance, the Hot Swap external 2.5" on the DF-85comes in real handy for transporting sets of AutoCAD drawing files.....Ive mounted a 7" x 7" x 7" CPU cooler w/o taking MoBo outta the case, love the cable routing, cooling and inlet filters, but fav thing is ability to fit a CPX form factor PSU.
PSU - jonnyguru said it best "
It is completely unmatched by any ATX unit on the market I can think of. You'd have to spend twice as much as this thing costs to find the next best thing, performance wise."
MoBo - With GPU's in SLI, th NF200 gonna come in handy as covered here
http://www.tomshardware.com/reviews/p67-gaming-3-way-sli-three-card-crossfire,2910-17.html
RAM - CAS 7 will come on very handy in AutoCAD .... also compare the results of CAS 6 versus CAS 8 here (up to 23% increase in minimum fps):
http://www.anandtech.com/show/2792/12
SSD - Vertex 3 stands alone at this point
HD - For AutoCAD might wanna upgrade to a Barracuda XT if budget permits
http://benchmarkreviews.com/index.php?option=com_content&task=view&id=708&Itemid=60&limit=1&limitstart=10
GFX - The 570 in SLI ($335 x 2) gets 873 total fps in Guru3D's gaming suite. The 560 Ti (900Mhz) in SLI (2 x $240) gets 862 fps.....The 560 Ti can easily be OC'd to 1000MHz .... here they needed to push voltage to max permitted, I got lucky and was able to do it w/o any voltage adjustment.
http://www.overclockersclub.com/reviews/nvidia_asus_gtx560ti/4.htm