i just did the upgrade yesterday on my last gen mac mini. from 1gb ddr2-667 to 4gb ddr2-667 (~3gb usable due to memory addressing limits). it was ok - not easy not hard. the hardest part is prying out the case. i had no plastic putty knife so i just used a metal knife. scraped the hell out of the aluminum shell. bent many hooks and almost broke one while trying to forcefully pry it out. once the shell is off, its just a matter of screws.
my advice is: pry it with a plastic putty knife so you wont scratch anything. pry it slowly and gently. it'll come out eventually. or just go ask the genius @ apple for help.
any ddr2-667 sodimm will do. try and get matching ones so dual channel will work. for example, if you want to max out the ram, 2 of this will give you 4gb total. in your mac mini, only 3.3gb will work due to hardware limitation.
http://ncix.com/products/index.php?s...o&promoid=1006
any sata 2.5 inch drives will do. for max performance, a 7200rpm will definitely make your computer faster.