I'm using Samsung 533MHz RAM, currently at 660MHz. I've boosted the RAM voltage +0.1V to achieve that. I'm also considering adding heatspreaders because those things are getting warm.
If you boost voltages, add cooling. I don't think liquid cooling on RAM is anything other than overkill, but your mileage may vary.
Also, your latencies are important. If you have to raise/loosen them, either add another 0.1V (+0.5V MAX!) or lower your FSB to tighten/lower them again. Keep latencies as low as possible, your performance will thank you.