Standard DDR2 800 is timed 5-5-5-15 at 1.8v so your first kit is factory overclocked. If you add the other kit timings will default to the slowest installed modules but you can probably overclock the new stuff to the same tighter timings as your old RAM. If not you will never notice in daily use anyway.
Any not reason to use this memory for my second pair of DIMMs?G.SKILL F2-6400CL5D-4GBPQ
1.8-1.9 V, 5-5-5-15 timings. Cheaper.
For $20 more, G Skill also has a 2 x 2 Gb pair
at 2.0-2.1 V, this one at 4-4-4-12.
I understand about the timings, but am still confused about the voltage. Is there any problem with running 2 sets of DIMMs with different voltage settings? Would both sets again default to the same, lower number?
Also, I assume the faster, lower latency memory would allow the existing RAM to run at its factory overclocked speeds. How much effect would this have on system performance? (I am running Win7 64 with a Q6600 CPU).