I would go for the E7400 because its higher clocked than the quad and it will run all the latest games with ease (plus it would overclock fairly well). However if you are thinking ahead to the future maybe the quad is the better option, who knows.
Bottom line though - I wouldn't invest too much into a socket 775 system anyway, theres no upgrade possibilites and you will be annoyed a year down the road I bet you when you wan't to upgrade your cpu and you can't. If that site has any am3 systems, grab one, as it will allow upgrades for atleast the next 3 years no doubt.