Your motherboard book should tell you if the connection for the harddrive are IDE or SATA or both
SATA drives are faster but IDE are the norm and slightly cheaper
I've been very pleased with Seagate and if you buy a "plus" model for about a fiver more, you'll get a 3 yr warranty
OEM harddrives often come without cables and software is usually downloaded from the manufacturers web site. Retail versions are slightly more expensive and usually have a cd disk of software and a cable. I wouldn't worry about buying OEM if you are as far as downloading from the web and ordering a cable over the net or from PCworld.
Cases usually come with the screws and fixings you need. Dabs and Novatech sell lots at reasonable prices