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...