Depends on many things. but generally you want a lot of RAM, a good multi thread CPU and for some things a good GPU.
does this seem pretty decent enough? I'm gonna hunt down the parts later-
Computer CaseCooler Master Force 500
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CPUIntel i5 3570K Overclocked to (4 x 4.6 GHZ) - Ivy Bridge
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CPU HeatsinkArctic Cooling Freezer 13
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Memory16 GB 1333MHz (2x8GB) - (DDR3) - Limited Time Low Price
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Graphics CardNVIDIA GeForce GT 640 - 1 GB - (ZOTAC) - (PCI-E)
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MotherboardGigabyte GA-Z77-D3H (Intel Z77) - VGA/DVI/HDMI
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Sound CardCreative Sound Blaster X-Fi Xtreme Audio 7.1 (PCI) (Limited Time Low Price)
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NetworkingMotherboard Integrated Ethernet Lan (Broadband Ready)
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Power SupplyCorsair 550W PSU - Low Noise
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CPU CompoundStandard CPU Compound Supplied With Heatsink
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Case FansStandard Fans Included With Case
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Hard Drive #11 TB Samsung 840 Evo SSD SATA-III, Read 540MB/s, Write 520MB/s - Silent
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Optical Drive #1Samsung 24x DVD/CD Re-Writer/Reader - Black - (SATA)
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Card ReaderInternal Card Reader 50-in-1
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Operating System #1Microsoft Windows 7 Home Premium 64 BIT (Genuine DVD & COA Included)
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Security SoftwareFREE - Bullguard Internet Security 2013 (1 Year - 1 PC License) - FREE FOR ORDERS PLACED BEFORE 30/11/13
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DVD/CD Burning SoftwareFREE - Nero V12 - NEW
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Monitor #121.5" HD LED Widescreen Monitor - Black - 5ms - 1920x1080 - DVI/VGA
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KeyboardStandard Slim Keyboard (PS/2)
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SpeakersLogitech (S120) 2.0 Speakers - Black
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Free GiftFREE - 16 GB USB 2.0 Flash Drive
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Monitor Cables & AdaptersFREE - DVI Port to VGA Monitor Port Adapter