k.. to tell you how much more you can overclock your system is really beyond me, only your own hardware knows... if you have really good hardware and can handle the overclock the limit is how you get there and watercooling is a great start for that
beware though, as i qoute a member from these forums "Once you go H20, you'll never go back" this has to be true, you definitly fall in love w/ it... anyhow.. on to your other questions.
your going to need (for a basic kit):
1. Water Block - cools the processor
2. Water block Clamp/Tie Down - holds the water block onto the processor
3. Radiator - cools the water
4. Radiator fan - cools the radiator which will cool the water
5. Pump - pumps the water threwout the system
6. Resivior - holds your water
7. Tubing - connects all the above together
and thats a basic kit... if you buy a kit you can get one
as cheap as 120.00 bucks or so..
notes: make sure if your buying a block and/or radiator from differn't places make sure you have the same size fittings (the inlet and outlets for each item) the same size common sizes are 1/2" and 3/8" you get alot more threwput w/ the 1/2" but i've never used a 1/2" system, i have only used 3/8" so far. gooduck and have fun