Well to answer your questions,
Processor
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If possible, get a Dual Core system
Ram
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PhotoShop eats this for breakfast. 2x1gb should be sufficient ( unless u intend to do large format printing
Storage
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2x74gb Western Digital Raptors should do the trick. Set them up in Raid0 configuration ( $305 each )
1x36gb Western Digital Raptor for your Operating System ( $186 )
3Ware 2 Port SATA ( $275 ) or some Cheapo SATA card ( $39 ). Forget onboard Raid. They tend to disappear under heavy or hot usage. I recommend the 3Ware model
Housing
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450W PSU = $105
If your casing is a cheapo $50, throw it away and MAKE SURE there is at least an 80mm or 120mm fan blowing at the hard disks. Else, go and buy a 2 bay into 3 bay HDD Expansion bracket. = $100+ or $45
Graphics Card
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Usually, a Matrox P650 128mb@ $479. Alternatively, you can make do with a ATI Radeon X1600Pro 512mb for $239
The above should suffice nicely, and you will feel the speed. Using only consumer components.
This is usually my recommendation for PS usage.
However, if your work is extremely important, I highly recommend you consider a Raid5 subsystem.
Raid 0 = Lose 1, Lose all. But fast speed.