You can check the memory with memtest86. Do a search for it and make a boot disk or boot cd. Run the program for 24 hours (yes really that long).
The only way to change the memory from single to dual is to remove one stick so one is installed. The motherboard will change this automatically and the bios I am familiar with there is not an option to enable or disable it manually.
Forget about concerns about dual-channel memory. Although your motherboard would support this, it's not possible since you're putting in two modules which don't have the same size. The other posters must have missed this in your original post. Your 1GB and 512MB sticks will only run single-channel.
Even if you got another 1GB module, it may not match the one you have now closely enough to run as dual-channel. Dual-channel is good because it makes your processor work with the RAM better, allowing it to address each "half" of it independently.
Your system does has the capability to work with two different sizes of modules - like what you have currently. You posted this information:
Currently Installed Memory: 1536 MB
Where did this information come from? According to this, your system IS seeing all the RAM in your computer. Where does it show less? Does the full 1536 show when the computer tests the RAM on Boot?
You might try swapping the RAM around. Try the 1GB in slot 1 and the 512MB in slot 2. Then the other way around. The BIOS does not need to be changed when installing RAM.
One easy way to see how much RAM your system sees is to go to Control Panel / System. The main General page there should show how much physical RAM you have.
Your BIOS has some very advanced options for adjusting Overclocking and controlling many features of your CPU and RAM. If the "System" results show 1024, then you can try changing some of those next.