Actually, mixing RAM is ok to do, if you do it right. Overall, I'd back up fatsauce on his statement that most people will tell you not to mix RAM, as it causes lots of frustration.
Back to mixing RAM. When you mix RAM you will:
Have all density (size), barring any defect
Operate at the highest latency of the mixture
have to operate the mixture at the highest recommended voltage of the mixture
have to check with the RAM manufacturer(s) to be certain their RAM can operate safely at the higher voltage (this applies to the lower voltage RAM)
For example: 1 x GB @ 1.7v w/ 5-5-5-15 & 1 x 1GB @ 2.2v w/ 7-7-7-18
In the above configuration, you will have to make sure that the module rated for 1.5v can safely operate at 2.2v, because the module rated at 2.2v will work best (in some cases not at all) at 2.2v. Furthermore, because the higher voltage module has a higher latency, you will have to set these timings in your BIOS, which means the 1.5v module is running slower than its potential.
Rules to Enable Dual Channel Mode
To achieve Dual Channel mode, the following conditions must be met:
Matched DIMM configuration in each channel
Same Density (128MB, 256MB, 512MB, etc.)
Matched in both Channel A and Channel B memory channels
OR
Populate symmetrical memory slots (Slot 0 or Slot 1)
Configurations that do not match the above conditions will revert to Single Channel mode.
The following conditions do not need to be met:
Same brand
Same timing specifications
Same DDR speed
Memory channel speed is determined by the slowest DIMM module populated in the system.
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