How can I tell if can increase RAM from 2 to 4 GB on my ASUS P5B MX motherboard?

"From newegg.com:

Memory
- Number of Memory Slots: 2×240pin
- Memory Standard: DDR2 667
- Maximum Memory Supported: 4GB
- Channel Supported: Dual Channel

I would recomend buying 2 * 2GB DDR2 667. This one in particular."

"The safest way to pick memory for your motherboard is to find the manufacturer's Approved Vendor's List (AVL). In Asus' case, it is called ""Qualified Vendor's List"" and is in the P5B-MX user's manual. If you pick memory from this list, then it has been tested and passed the motherboard vendor's qualification. Unfortunately, this list can be dated. So here is my criteria to use for selecting memory:

For P5B-MX, you definitely want DDR2-667 memory.
Also, avoid CAS Latency = 3, which apparently downgrades to 533 due to chipset limitation (at least at the time the User's Guide was written). NOTE: Generally the lower the numbers in the timings, the better.
Select memory because...

It is the specific module listed on the AVL.
It is close to the specific model listed on the AVL (minor revision change).
It is from a manufacturer/has same components as model on the AVL.

Other good options:

Recommended memory form someone who used the specific module on their P5B-MX motherboard.
Memory on the AVL of another motherboard with the exact same memory controller and configuration.
Quality memory that fits the specifications from a tier-1 memory vendor (usually not the cheapest).

Of course, there is always risk that the memory will be bad. I suggest once you get your memory, even if you use the AVL mentioned above, your run it through a good memory test. The best memory test tool I know of is RST-PRO, but it costs money. You can find some adequate memory testing programs online, like Memtest86, for free."