That setup would be fine for Skyrim. P67 is fine but I'd go with a Z77 board (you might as well get the latest chipset) and an i5-3570k (or a i5-2500k if the price was right). I wouldn't bother with the 6790 - The radeon 7850 is priced very well to sell and it's a nice powerful card. There are 2gb and 1gb versions. Your PSU should handle it fine.
A 500w psu will power a modern 28nm graphics card as strong as a GTX670 or 7950.
Assuming it is a quality unit with 2 6 pin connectors, you can reuse it.
P67 is a bit outdated. Z77 should cost no more and has some added advantages.
Plan on $100-$150.
Times have changed. Your $999 dual core was top of the line some time ago.
It had a passmark rating of 1918.
Today, a i3-3220 which costs $120 or so has a rating of 4274.
Today's top gaming cpu is the 3570K quad core with a rating of 7142, and that is before a conservative overclock from 3.4 to 4.3.
And... no water cooling is needed. A $30 hyper212 will do the job.
Raptors were wonderful in their time.
But today, any SSD will be 50x faster in random i/o and 3x faster in sequential.
No raid needed.
Ram is also cheap. A 8gb kit of 2 x 4gb DDR3 1600 will be $40.
Your old DDR2 is actually more expensive and has a resale value on ebay.
If you have not upgraded to windows 7, I would do so. I think I wouuld bypass w-8.
Put together a list of parts and links so we can see what you are thinking.
Budget sucks just as bad as the Social Security system does.
I really hate to give up this system. I love this Intel d975xbx board, and the dual core processor.
So, having said that. I'm not going to cannibalize it to the point that it is not able to run independently. I'm putting together a new one on pcpartspicker.com, and I'll post the link when available. Each of you has given me a different angle to think about, and every one of them is appreciated.