I had 4GB of RAM and a 120GB HD. Both were as received from Apple. For a couple of years I was running my boot drive from an external FW800 Raid Enclosure using RAID 1with 500GB capacity. The enclosure bit the dust recently so I was then running off my USB 2.0 clone drive that I keep updated. Every time I do any OS update I use SuperDuper to make an bootable clone of the boot drive. (btw I also use Time Machine for ongoing backup). I wanted higher performance but didn't want to buy a new computer so I decided to upgrade the drive with a WD Scorpio Black 750GB HD and while I was at it, upgrade the RAM with 8GB of Kingston RAM. (I also run Parallels so the extra RAM is a plus).
Following IFixit's guide for MacMini A1283 HD and RAM replacement and using the tools obtained from IFixit I dove in and did the replacement. I was nervous at first, but following the guides I gained more confidence as I went. All went perfectly. My 2009 vintage Mac Mini runs like a champ. It's like a new machine!
A tip I would give would be for the HD replacement. At step 13 I did the skip ahead to steps 18 & 19 then returned to step 15. I found removing the heat sensor from the drive was far less dangerous and easier than fiddling with the thermal sensor connector.
Also, before replacing the HD, I would suggest doing a fresh install of OS X on the new drive then let the OS migrate your data, boot and do a software update and boot again. That way you know you have a good bootable drive before you tear into the Mac Mini.