what upgrade would be great for a hardrock sport 2004
It depends what you want to do with the bike.
If MTBing, disc brakes, preferably hydralic ones. Wet weather power is awesome relative to V brakes. Just check wheels and forks are disc compatible.
Disc brakes are really the last thing I would do. Assess what is the weak point of the bike, and then go from there. If you have to ask what to upgrade, you probably aren't ready to really upgrade anything, since as you ride it will become apparent what the "weak links" are in the design of the bike.
I agree with both of these Points, i mean, as you are from the UK, an obvious weak link would be how a set of V brakes performs in our ......... damp conditions lol, If i was to upgrade a part of a Hardrock sport, judging on our weather, i would upgrade the disc brakes, possibly to Avid mechs, or if you want to go hydrolic, i would look at either a set of magura julies, because imo, these are excellent budget disc brakes, or if you have a lot of dosh to throw around, get a set of Hope mono minis or hope mono m4, i have a julie on the front and a m4 on the back, and apart from the heat difference that is lost between the front and the back, i cant tell the difference....
Also, i may consider upgrading the fork, but based on another thread that you made, we've already discussed that...
Work hard, Play hard
IMO I would upgrade:
Rear Derailleur(Preferably to XT)
Fork(already been discussed)
Wheels(something disc compatible)
Brakes(hydro discs like the ones Cornish mentioned)
Replace stuff when it breaks. Not just for the heck of it.