Can I mix & match RAM like this combination
 

Hi guys

I already have 2 sticks of Crucial Ballistix 2GB (2x1GB) DDR2 800 PC2-6400 running on my PC. If I would like to upgrade my RAM from 2GB to 4 GB, can I mix & match & add 2 sticks of Crucial Ballistix 2GB (2 X 1 GB) PC2-8500 DDR2 2GB DDR2-1066 CL5-5-5-15 Memory Kit to my current PC ?

If yes, should I install the faster speed at slot 2 and 4 and leave the slower speed ram closer to the CPU ?

Please kindly advise ASAP.

all of them will just run at 800mhz

It generally isn't a problem, but there's a small chance they will cause problems like BSODs, random freezes, etc.

Run the memory tester built into Windows Vista or Windows 7 after installing to make sure they'll work fine.