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You don't really need to rinse it but I suppose it doesn't hurt. I used to rinse my new rads but nothing but clean clear water came back out so I've stopped doing it. But to be on the save side yes you can use boiling water and vinegar and then rinse it out with distilled water.
Defintely get a bigger SSD drive, I recommend at least 80gb. Raptor technology is a bit dated, get a 256gb ssd if you can afford one or a 7200 RPM drive like the WD Black, you'll get more storage and the speed difference won't be that different.
It's the 2nd best (or the best, depending which review you are reading) air cooling heat sink money can buy, so it will give you peace of mind if anything if you buy it.
http://www.overclockersclub.com/reviews/antec_kuhler_h20_620_review/4.htm
They both perform almost identically. I'd pick the one you like the best aesthetically. However from my experience I found the fans on the H70 to be like a jet engine, but you can replace them with Gentle Typhoons for...
Did you manually set the RAM timings in the bios? Than can help. Also turn off the turbo profile and put it to normal. Disable speedstep, c-state and cie. Turn on extreme phase power mode. If that all fails you can try adjusting the clock skew manually.