I've also read articles suggesting that there could be more. Pure theory though and I don't know how it all stacks up. Certainly the Standard Model only has the three generations for all fundamental particles. I'm a firm believer that as an amateur astronomer, I'll read these atricles with interest, but it's safr to stick to what is currently accepted physics until it's proven otherwise.
Wikipedia also has a good description of quarks and can be found here:
http://en.wikipedia.org/wiki/Quark
Cheers,
Andy.