How much RAM do I really have?

I have an 8 year old MESH with an A7V8X motherboard.

I recently started finding it slow so I bought an extra 1GB RAM to go with the 512 it came with. I have installed the extra 1GB and its definately faster.

However, the manual says:

3 x DDR DIMM Sockets
Maximum 3GB unbuffered PC2100/1600 non-ECC SDRAM Memory.
Note: PC2700 maximum to 2 DIMM support only.
PC3200 maximum to 1 DIMM support only.

The 1 GB RAM I bought is PC3200. So does this mean that the most RAM I can ever use is that 1GB and that the (old) 512 is redundant and may as well be removed?

When I look at My Computer/Properties it says I have 1.5GB RAM. Also SpeedUpMyPC software says I have 1.5GB but are these just telling me how much RAM I have installed rather than how much of it my PC can use at its max?

How can I test if I can use the full 1.5GB?

Thanks.

Addendum...

Now my PC keeps crashing!

After 5 mins I get the black screen message "We apologize for the inconvenience, but Windows did not start successfully. A recent hardware or software change might have caused this." My options are:

Start in safe mode.
Safe mode with networking.
Safe mode with command prompt.
Last known good configuration setting (most recent setting that worked).
Start Windows normally.

Ive tried the last 2 but it then gets stuck in a reboot-crash cycle and the only way I can break it is to switch the PC off at the back of the CPU. I cant get the the safe mode options.

Could this be because Ive left the old 512 RAM in??? :cry

 

Try taking out the 512 stick and see if it still crashes. Sometimes different brands and speeds of RAM don't play well together. MemTest would be a good choice as well. Get the one that you boot off a CD.

 

Your motherboard only supports one PC3200 module, but if your 512MB is PC2700 or PC1200, it will force the 3200 to clock itself down and the board will run two PC2700 modules, which it can do. That's why you seen them both in the BIOS and in XP.

Fred already suggested step one: Take out one of the memory sticks and see if the crashes stop. If they don't, put that one back in and take out the other one instead.

Optional step three: RMA the bad memory stick.