I have found when building boxes for my engineering (CAD) business, that a hi end box (say one tier down from top end) will last about 4 years. It does become a hand me down tho, going from the CAS table to an Engineer's box to a secretarial station as it "goes thru life" making it last a bit longer.
Like a gaming box, one of the 1st decisions is just how big on GFX do you wanna go ? Single card, multiple cards ? ..... You going to be using PhysX plug ins for Maya / 3D Studio Max ?
http://www.geeks3d.com/20081207/physx-plugins-for-3d-studio-max-and-maya-available/
Then that's another consideration....back to GFX later.
Case / PSU - Hard to look away from the Antec 1200 / CP-850. ($250 for both at newegg)
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129043
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371024
http://www.newegg.com/Product/Product.aspx?Item=N82E16835209011 (Extra Case Fans)
MoBo - Usually a gamers MoBo, the ASUS P6T7 WS Supercomputer gives you three GFX card slots + a 4th for a dedicated PhysX card. Unless you're looking at 3 or more cards, I'd recommend the ASUS P6X58D Premium which provides SATA III and USB 3 support but only PCI-E x 16 lanes (16 x 16 x 1 or 16 x 8 x 8). Lowest I'd go for is the ASUS P6T Deluxe V2.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131390 ($400)
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131614 ($310)
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131365 ($290)
CPU - i7 obviously but what you buy is strictly a budgetary decision. The 920 is a great CPU but fact is time is money and though the 920 can be OC'd to 975 speeds, the 940 is faster, each step up is also faster. Go with the max you can afford is box will be used in a production environment.
Memory - 3 x 2GB is the starting point. Does your work demand more ? DDR1600 is a great choice for it's OC ability, no need to go higher unless you looking for 210+ BCLK's . Gran the lowest CAS # you can afford at DDR1600. Best right now is Mushkin 998692's w/ the Ascent coolers w/ CAS 6 (6-7-6-18) if you can find them (but can only use 3 modules cause of the cooler's width on most MoBos. The sister modules (998691) w/o the coolers are back on newegg. The cheapest CAS 7 seems to change every few days.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226050 ($245)
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145286 ($180)
Hard Drives - Check out the performance charts and pick whatever 500 GB per platter drive performs best under your usage patterns. The WD Black 2 TB is a 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-hard-drive-charts/compare,1006.html?prod[2371]=on&prod[2770]=on)
SSD - If ya budget can afford it, get one. Intel G2 series or new OCZ Vertex2 if it hits the streets
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=Intel+G2&x=0&y=0
Optical Blue Ray Reader or Blue Ray Writer ?
Plextor Model PX-B320SA Blue Ray Reader / DVD Writer w/LS OEM
PLEXTOR r Model PX-B940SA SW Blue Ray Writer
http://www.newegg.com/Product/Product.aspx?Item=N82E16827249055 ($135)
http://www.newegg.com/Product/Product.aspx?Item=N82E16827249057 ($280)
Heat Sink - For performance and ease of installation, the Prolimatech Megahalems fits both criteria. Here's what I'm putting in new builds Mega w/ IC Diamond TIM and twin Scythe PWMfans (make sure ya MoBo can handle the fan wattage). ($95 for the HS, TIM. two PWM fans and a Y cable splitter))
http://www.frozencpu.com/products/8807/cpu-pro-01/Prolimatech_Megahalems_Rev_B_Intel_CPU_Heatsink_LGA_775_1156_1366_AM2_AM2_AM3_Hot_Item.html
http://www.frozencpu.com/products/7038/thr-41/Innovation_Cooling_Diamond_7_Carat_Thermal_Compound_-_15_Grams.html?id=BZWnrfIC
http://www.frozencpu.com/products/10026/fan-639/Scythe_Slip_Stream_120mm_x_25mm_PWM_Fan_-_SY1225SL12LM-P.html?tl=g36c365s936
http://www.frozencpu.com/products/8418/cab-150/FrozenCPU_PMW_Y_Splitter_Cable.html
If that's a budget breaker, look at the Xigmnatec S1283 or one of the others here:
http://www.silentpcreview.com/Recommended_Heatsinks
http://www.frostytech.com/top5heatsinks.cfm#INTELHEATSINK
http://benchmarkreviews.com/index.php?option=com_content&task=view&id=371&Itemid=62&limit=1&limitstart=15
GFX Cards - Though plain ole CAD is a staple here, I don't get much call to build 3D Workstations, so I'll have to beg off on recommendations here since I'm not keeping up w/ the latest and greatest. Like w/ the HD isue, the selction of best card will depend on what program. The last THG article I read had the nVidia winning the Quadro FX 4800 Editor's Choice designation but Maya specifically favors ATI cards. Compare your usage with the benchmarks here to make a determination.
http://www.tomshardware.com/reviews/quadro-fx-4800,2258-11.html