How much can I rise the voltage (Corsair XMS2 PC2 6400 CL5)

Hi to everyone in the forums. I´m completely noob in OC'ing and recently change my system to the next spec's:

Motherboard: Biostar TP43D2-A7
Processor: Intel Core2Duo E8400
RAM: 4GB (2x2GB) Corsair XMS2 PC2 6400 CL5
Video: PNY 9800 GX2
PSU: OCZ StealthXtream 600W
Cooler: OCZ Vendetta

I OC'd by changing the CPU frequency setting from 333 to 380 MHz (3.420 GHz processor and 912 MHz memory) but in the process I had to change the CPU voltage to +5% and the memory voltage to 2.1 V (1.9 by default).

Then I go to 400 MHz CPU frequency but the system didn´t boot unless I change the memory voltage to 2.2V so this are my questions:

1. How much can I rise the voltage without damaging the RAM?
2. I read in some guides that is risky to increment the voltage more than 15%, is that true?
3. What is the most higher frequency I can get with this ram? (400 CPU Freq = 940 MHZ RAM clock)
4. Should I need to change the RAM timings? (right now I'm at 5-5-5-15)

Thanks in advance for your comments.

In ASUS M3N-H/HDMi there is no option for changing RAM voltage or timings...