I generally don't recommend AMD based systems unless budget limits are down around $1,000 territory. AT $1200, I'd do 1156 based Intel and at $1500 a top end 1366 based system is more than feasible, a top end 1366 system is possible.
Goal here was to provide for a system capable of a 4.0 - 4.2 GHz Overclock w/ little effort and provide enough oomph for adding a 2nd card in SLI. Case provides stellar cooling, PSU can handle all but twin 480's / 5970's and everything else is top shelf.
You could use the old case, but budget allows for an improvement there and not sure if the 900 will fit a nice big CPU cooler.
$150 Antec 1200
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
$120 Antec CP-850
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Dang .... if not in a hurry, newegg usually has a combo on the two above for $220 - $240
Alternate ($60 more) would be:
$ 140 HAF 932
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
$ 190 HX850
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
$514 ASUS P6X58D-E w/ Intel i7-930
http://www.newegg.com/Product/ComboDealDetails.aspx?Ite...
$155 3 x 2GB Corsair DDR3-1600 CAS 7
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
$80 Seagate Barracuda 7200.12 1TB
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Check out the performance charts and pick whatever 500 GB per platter drive performs best under your usage patterns. The 2 TB WD Black and XT from Seagate are good choice but at smaller capacities, you are limited to the Seagate 7200.12 or the Spinpoint F3. The 7200.12 excels in gaming, multimedia and pictures whereas the F3 wins at music and movie maker. See the comparisons here (copy past link in manually, link won't work in forum):
(
http://www.tomshardware.com/charts/2009-3.5-desktop-har...[2371]=on&prod[2770]=on)
Look at the tests that reflect your usage and choose accordingly.
$101 Prolimatech Megahalems w/ twin PWM fans and IC Diamond TIM w/ PWM Cable splitter.
http://www.frozencpu.com/products/8807/cpu-pro-01/Proli...
http://www.frozencpu.com/products/7038/thr-41/Innovatio...
http://www.frozencpu.com/products/10026/fan-639/Scythe_...
http://www.frozencpu.com/products/8418/cab-150/FrozenCP...
$25 Any DVD Writer
$350 ASUS ENGTX470/2DI/1280MD5
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Hard to pick the choice here. If ya spend more money (+350), you could get a 5970 but twin 470's in SLI has much better minimum frame rates than the 5970 in DX11 games (1920 x 1200, hi settings) by about 23% (17% for avg. frame rates) .... could add the 2nd card come XMas time.
http://www.hardwarecanucks.com/forum/hardware-canucks-r...
http://www.guru3d.com/article/geforce-gtx-470-2-3-way-s...
http://www.softsailor.com/news/22371-nvidia-geforce-gtx...
Another option would be to get a 5850 and add a 2nd at Xmas....will save you $50 per card ($14%) but will also drop min. frame rates by about 19%. Each card is well priced for it's performance in the market so hard to find fault with any of those choices. This list below which provides the cost in "dollars per frame" (avg of 9 benches in DX11, 1920 x 1200 @ hi settings) might help ya choose.
ATI 5850 8.89
nVidia 470 9.08
nVidi 480 10.01
ATI 5870 10.08
ATI 5970 11.51
The ATI cards fare better in DX10 so if that's where you'll be spending play time, I'd go that route.
Total $1,495
Either way, can keep your old card as a dedicated PhysX card. Again, your 285 can do your current resolution but again, your budget allows for improvements. If you just wanna keep the 285, can delay a GFX card upgrade and grab a SSD.
$235 Intel G2 80 GB
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Reusing the case not upgrading the GFX card would save $500. If ya grab that SSD, you still have $265 to spend or can get bigger / better SSD
http://www.newegg.com/Product/Product.aspx?Item=N82E168...