Great idea building the machine w/ ya son..... I have got 3 boys and we've done 3 builds each together. Great way to get some father / son bonding over common interest. Be advised, not only have I done theirs but at this point, I'm doing all their friends and their friends parents as well
here's some data:
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
$ 210.00 6950 (479/751) $ 0.44 - $ 0.56
$ 260.00 6950 Frozr OC (484/759) $ 0.54 - $ 0.69
$ 205.00 560 Ti (455/792) $ 0.45 - $ 0.52
$ 360.00 6970 (526/825) $ 0.68 - $ 0.87
$ 205.00 560 Ti - 900 Mhz (495/862) $ 0.41 - $ 0.48
$ 320.00 570 (524/873) $ 0.61 - $ 0.73
$ 480.00 580 (616/953) $ 0.78 - $ 1.01
Pour over it with ya son..... good lessons (and math problems) there.
As you can see, the 900 MHz 560 Ti has the lowest cost per frame both as a single card and paired in SLI. More importantly, we see that $410 dollars buys 873 fps where as the top of the line 580 gets ya only 616 for $480 .... good lesson there. Personally, I prefer the Asus model as I have had great luck OC'ing them up to 1000MHz...worth the extra $25 IMO.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121425