A good site for EKG stuff is www.emergencyekg.com
It's the site associated with the book "Rapid Interpretation of EKG's," by Dale Dubin MD...the gold standard of EKG info out there, according to several of the cardiologists I've talked to.
There is also a site which I think is run by someone on this BB, www.icufaqs.org
which has a lot of good info.
is a website for emergency medicine/primary care...I know they have EKGs and CP scenarios...not sure what else they have about the heart...but it's just a plain cool site. You can look at pics of xrays, cat scans, and other fun stuff.
You could also go to yahoo or one of the other search engines and look for "cardiac nursing," or "the heart," and probably find some good information.