Sorry man, you're throwing money away. I don't know how much you're thinking of spending doing the upgrade to that older box, but it's a waste of $$$ to do it.
Go look for a new I3 Sandy Bridge unit with 8 gig of ram and a decent Video card with 1 gig ov Vram
Might add a GE Force video card to this: http://www.tigerdirect.com/applications/searchtool
If you don't get a new box, you're just crippling yourself for any future release of ADSK products. If you can go as high as $1500, look at my sig.
Last time I checked for version 2011 plain AutoCAD and Map3D are single processor applications. Except for very limited instances a multi-core processor won't help you. I'm not sure about the Civil3D product.
So with a 64bit machine, I would put together a machine with at least 8-16gig ram and the fastet video card you can get with 1gig of ram dedicated to the video card. DO NOT pick a machine that uses 'shared' or similar memory.
Just my $0.02
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