DX9 has been around forever now, so don't let him sucker you into a $500 video card...I've seen it over and over again and I always wanted to say something, but the one time I tried the Mom looked at me like I was crazy...
Anyhoo, just go visit newegg or tigerdirect or microcenter or frys online (all dot-com, of course) and look for a low-powered card (in the "Details" or specs it will say something like TDP 100w and look for something equal to or LESS than this). Virtually any card in the $80 to $100 range today will be worth its salt and certainly capable of running DX9, and they will all have at least 1GB of ram, no worries.
Just look for big numbers except in two areas: TDP and the Price! Because believe it or not, everyone else does the same thing...more or less! It will all be good in the end, and your boy will love you for it. Tip: Radeon 7000 class a very good buy.
Of course if you're looking for a complete PC then go with Dell, HP...or if you're willing to spend $100 more, check this out...
http://www.newegg.com/Product/Product.aspx?Item=N82E16883229437
A six-core processor with a 2GB Radeon 7750, a 1TB HDD and 8GB of DDR3, for under $600. But if he doesn't want it you can always buy it for me!
^D