The 760 uses Non-ECC dual-channel 667 or 800MHz DDR2 SDRAM with a maximum of 8GB (4 x 2GB) or 4GB (2 x 2GB) in the USFF (ultra small form factor)
If you mix frequencies of RAM it should default to the lower speed and you probably wouldn't notice the difference in performance if you're just using the system for typical everyday tasks. You can replace the factory installed memory altogether if you want, it doesn't matter, as long as the memory you replace it with is compatible.
I hope this helps clarify things.
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