Need to know what HDD my PC needs

I own an HP Pavillion 8566c. I need to replace the HDD. I don't know what HDD's will work in it. I think maybe ATA drives, but I'm not sure and I don't know which ATA drives will work in it.

Here is a link to info on this PC's hardware:http://h10025.www1.hp.com/ewfrf/wc/product?lc=en&dlc=en&cc=us&product=60975&

andhttp://h10025.www1.hp.com/ewfrf/wc/documentSubCategory?tmp_rule=14452&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&product=60975

This site claims these HDDs will work in it, but I don't want to buy from them:http://www.drivesolutions.com/cgi-bin/shop/ugdtstore.cgi?command=listitems&kind=hpdt&pos=0&type=itemid&itemid=hpdt1601

Do I need to worry about the PC not working with certain size HDD's (the amount of gb)?

What about ATA/#? I see ATA/66, ATA/100, etc does that matter?

Will this drive work in my PC?http://www.amazon.com/Western-Digital-7200RPM-Buffer-3-5INCH/dp/B000O9T5IA/ref=sr_1_12?ie=UTF8&s=pc&qid=1275798027&sr=1-12

Would I be better of buying one of these?http://www.newegg.com/Product/Product.aspx?Item=N82E16883103228

Does anyone know?

I would strongly recommend getting a new computer otherwise most hard drives for desktops are 3.5 inches and the amount of gb just determines how much you can store on it and it doesn't affect compatibility.