Actually, I just added a 200 GB Seagate HD (on sale for $99
) to a 400 MHz P2; it worked fine! Mine came with all sorts of tips though (but no controller card), and I'm sure you could find some option that will work. Also, make sure your OS can support it (Linux can...and yes, at least half of my posts are going to mention Linux); I believe only XP and 2000 natively support giant hard drives, but even if you have one of those, you may still have problems. Sadly, determining compatiblity isn't as easy as knowing your processor. This page may help:
http://www.seagate.com/support/kb/disc/faq/137_overview.htmlBTW, you could just buy a controller card; that would work (with XP or 2000 only though probably), but why waste money if you don't have to? Check out the above page, and try a google search.
Good luck!
edited to add specs