I plan to buy GTX 280 and since I've heard about it's fan noise, and since i would like to OC it, I plan to water cool it with Zalman reserator I am about to buy as well.
I saw lots of cards which you can buy overclocked already which are very expensive, and my question is, can i just buy the simplest and cheapest one, replace their existing fan cooling with a VGA water block, like EK's , and get the best performance?
Can't answer all the questions but should be able to bring to light a few of them.
I would go with Evga for your 280, that way you can step up if new arch's come out within a couple months. As for buying any of the OC'd versions, from what I've read, the company takes all their cards and test them, finds which ones can run at higher clocks and OC's them and sells them for a higher profit for the same card. If this is true, and you buy a non-OC'd GPU, it could be a bad overclocker and they decided to sell it as the "regular" card. I would think that any 280 you buy will overclock nicely on water, so worry not about getting a good boost from watercooling vs stock air cooling, water should allow good OC'ing ability.
The tubes and fittings you need all depend on the kit. A half inch setup will cool better then a three eights however there seems to be fewer half inch parts. Also you may find bending half inch tubes a bit harder, I imagine they would be more prone to kinking.
My suggestion to you is put up the extra for a factory overclocked card. If you are buying on the cheap I imagine your gonna get a pump that is gonna make noise. After your done buying your watercooling equiptment you would have paid the difference anyways.