Well, I bought my 2nd hard drive from Sam's Club and have had no trouble with it. I can't remember the brand though. I would suggest finding out what type you need(which it sounds like you know) and shopping around a little online to find it the cheapest. Sometimes TigerDirect.com has decent deals. Also if you find a brand you want, check Epinions for any customer reviews.
I have an 80G that came with the computer, and bought a 160G to supplement, and plugged it in, formatted and it has never given me a problem. What I do is run all my programs off C drive, and use my secondary 160G drive to store big stuff(I do a lot of graphics/video work). I would definitely recomend a second internal since you are all set-up for it!!
One slight word of caution, if you are upgrading your RAM, keep in mind you may lose the ability to run a program or 2. I upgraded from 512M RAM to 2G and my best graphics program would not run, because I had TOO MUCH RAM!! Hard to believe, but it can happen.
....so now that you are totally cornfused...!
