Thats a pretty tight budget to work with. I use Canada Computers for general hardware here in Ontario (if you know of some other more affordable stores please tell me!!). For the proc you looking at around 240 for the 1.8ghz one, mobo lets say 260 bucks, video card lets say another 150 (if all he plays is aoe3 then thats fine), Antec case with psu around 120, dvd combo drive 50, hd another 100 dollars, memory another 200, xp home 100, and 200 minimum for a monitor. Now add that all up. Its like double your budget
You probably could get something for around 700 but under that is hard with a dual core proc.
PS whats your dads username? Id like to play him sometime