^ It depends on what you want to do and how much you want to spend on the PC...
Here are a few build options -
Intel - i5 750 - It is a very good value for money in terms of gaming performance and easily keeps up with the i7 930...But if you are planning for SLI/ Crossfire high powered cards like the HD 5870/ GTX 4x0, then you would have to certainly overclock the CPU to avoid any CPU bottleneck though...
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Mobo -
ASUS P7P55D-E Pro
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
RAM -
G.SKILL Ripjaws Series 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Intel i7 930 Build - IMO if Crossfire/ SLI is for sure, then I would go with this platform if money is not much of a constraint...
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Mobo options -
For 2 cards, you wont need those mobos...
Get any of these...These are very good and cheaper...and has all the options what a mainstream users might want...
http://www.newegg.com/Product/Productcompare.aspx?Submi...
RAM - The one you have chosen is very good...stick with it if you will be getting this platform...
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Now for AMD -
If you to save some money, then you can get the AMD CPUs too...
CPU -
If you will surely overclock -
X4 955
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Might not, then get the fastest Phenom 965
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
As for the 6-core, they dont offer any significant gains in gaming...So suggesting them would only depend on what else do you do with the PC apart from gaming...If the programming that you do involves the maximum utilization of the CPU cores, then the 6-core might be a good choice, else I dont think it would be a good option...
AMD Phenom II X6 1090T
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Mobo - Though even the 890GX chipsets would do, but if you will be getting 2xHD 5870, then the 890FX with full x16x16 would be a better choice...
ASUS M4A89TD PRO AM3 AMD 890FX
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
GIGABYTE GA-890FXA-UD5 AM3 AMD 890FX
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
RAM -
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
And if you will be going with AMD, then you would have to HD 5870 as these boards only do Crossfire...
XFX HD-587X-ZNFC Radeon HD 5870 1GB
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Common parts -
PSU - If you will be getting only 2 cards, then get a 850W PSU...It can handle those...
Modular
http://www.newegg.com/Product/Productcompare.aspx?Submi...
Non-Modular
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
HDD - With that budget, you can get a SSD for OS and apps and a regular drive for data...The SSDs offer very good boot times and apps start up is very speedy...
For OS
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
For data - Samsung F3 500GB
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
And CPU cooler -
CM hyper 212+
http://www.amazon.com/gp/product/B002G1YPH0/ref=s9_simh...