Air cooling is pretty limited in what you can do.
Larger fans (120mm) push more air at slower speeds so they are more effecient and quieter. Most cases are fit for 80mm fans so if you plan on using a 120mm case fan you'll have to get out some tin snipes or a dremel to make room.
You'll want a good heatsink and fan on your cpu with some thermal paste. I am personally partial to the Zalman family, they make some good combos that are good at cooling and quiet at the same time.
To answer your question, yes when liquid cooling from time to time you must top up (like any system some is lost to evaporation.)
AMD Athlon 64 3000+ (Overclocked to 3300+)
1GB PC3200 DDR
2x200GB Seagate SATA RAID 0
BFG 6800GT OC w\\128MB Ram
2x 19\" Samsung 930B LCDs