The best upgrade I ever installed is Solid State Drives (SSD) - I have one in my BI pc - recording to it, etc., and also have an external eSATA 2Tb drive attached for offloading video before archiving.
I'm using Crucial SSDs - been using them for around 3-4 years now (purchased when the price was way higher :nuts: , but I've had use of them also, so it works out). It would be really difficult to go back to traditional 7200 RPM drives - I had to do a recovery a few months back to one and I could not believe how slow it was by comparison to an SSD.
On most stuff I think refurb is fine - the one exception is hard drives (or SSDs).