I'm currently running dual channel OCZ DDR2 800MHz 2 X 512MB RAM. I intend to upgrade to 3GB for some light gaming purposes and with the possibility of upgrading from my current windows XP OS to windows 7.
My questions are
1) Would 3GB be sufficient on Windows 7
I'm having difficulties finding the same type of RAM from the ones i'm currently using (OCZG8001GK). I'm looking at these Mushkin sticks which have the same voltage at 1.8V but different tRAS ratings. The OCZ one is 15 (5-5-5-15). The mushkin ones state 12 (5-5-5-12) but from the reviews are 18.
You will not be able to do dual channel with 3GB! Both sticks must be the same size and have the same timings in order to do dual channel. The brands don't have to be the same, but if the modules are different, you will likely have significant stability issues.
Get two new sticks in a 4GB (2x2GB) kit. Or, just get 2GB in 2x1GB. The timings don't need to match what you originally had, nor do the frequencies. Just get what's on discount, and as long as the kit is DDR2, you'll be OK.
1) 3gb is fine for windows-7
2) If you put 1.5 gb on each channel, you should still be able to run dual channel.
I do not recommend mismatched ram. Some motherboards are very sensitive to small manufacturing differences.
Instead, I suggest you market your old ram on e-bay or wherever and buy a 4gb kit(2 x 2gb)
It is easier for a motherboard to manage two sticks vs. 4.
Go to the ram vendor's web site, and access their configurator.
Corsair, Kingston, Patriot, OCZ and others have them.
Their compatibility list is more current than the motherboard vendor's QVL lists which rarely get updated.
Enter your mobo or PC, and get a list of compatible ram sticks.
Cpu performance is not very sensitive to ram speeds.
If you look at real application and game benchmarks(vs. synthetic tests),
you will see negligible difference in performance between the slowest DDR2 and the fastest DDR3 ram.
Perhaps 1-2%. Not worth it to me.
Don't pay extra for faster ram or better timings unless you are a maximum overclocker.