1. Is corsair more popular and reliable than GeiL and what does kingston stand against those two brands?
2. Say a memory is stocked clocked at 675mhz and its specs are DDR2-667 (PC2-5300) does that mean it will be less compatible with the motherboards that support 667mhz?
Corsair has a better rep than Geil - yes. I have had mixed results with Geil: some good, some poor. I have had to return about 8 sticks in total of Geil RAM - mostly their low latency offerings, purely because the sticks would not pass 24 hours of MEMTEST86 at their stated timings as set by SPD.
It is highly unlikely (in my view) that you will EVER get GEIL RAM to operate at ANY speed faster than the SPD settings. GEIL seem very agressive in this regard. Corsair are far more conservative, and you are therefore more likely to be able to eek out a cycle or two in some of the timings.
Corsair is more expensive though, so that is not surprising.