Ok, the main factor is the rad size.
Rad: To beat a decent air cooler you will need a 240 or 360 rad.
Pump: cheap pumps transfer heat into the loop, but a decent size rad will sort this. Any pump is fine for a cpu only loop. If you are cooler mb and gfx card a better pump is required.
Cpu block: Any water block is good, from EK, D-tek etc, most blocks are within a few degrees C of each other.
Obviously, the cheaper the parts the reduced temp gain over air cooling. IMO the 45nm and 32nm chips of today do NOT get hot enough to warrant water cooling. The 90nm chips back 3-4 years ago needed decent cooling for a good overclock. These days the chips are effected more by the cpu voltage and electromigration before heat becomes an issue.
My 2 cents: don't waste money on watercooling, yes it looks bling but a good aircooler is cheaper, safer, easier to fit, more practical and cools the cpu well enough for a 4-4.5ghz overclock.
Aries 2010 Oc3D Winner
E8400@4.5Ghz 1.4v, Asus commando, HD4870 512mb @ 835/1020, 4gb ballistix @1011, 188.8.131.52, Water, avec thermochill and fuzion, Zalman 850w Braided, Yuraku 24" aou A-MVA panel,
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