Mmmmm, DirectX10, you say?
This is tricky. You see, the two DX10 architectures at the minute are, well, stupid.
The 8*00 series from nVidia performs exceptionally well in DX9, trouncing ATI's 2*00 series. However, there are some reports suggesting the 2*00 series from ATI counterbalances this with a superior DX10 performance.
However, that said, I'm recommending the nVidia option.
Below is a list of parts I'd suggest for a nice, high end system.
Note that I've opted for only 2GB of ram, since you won't use 4GB in an x86 operating system (3GB maximum, unless you disable the page file).
Also note the use of one large hard disk, as opposed to two smaller in a RAID configuration, and the presence of a Blu-Ray reader.
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