In a 32 bit OS, there are only 4GB of memory addresses in total, (2 to the 32nd equals 4GB). The hardware addresses are then subtracted from the 4GB, and what's left is, "available RAM. So, the more hardware, the less RAM is reported.

The OS uses the RAM also so that is subtracted also, but those processes are visible in task manager.

Task manager reports "available RAM" in >kilobytes< not >megabytes<. the conversion ratio is 8 to 1, not 10 to 1 as we're accustomed to in decimal math. Hence the number in MB is going to appear smaller when the conversion is made.

4,4,4 12 speed is very similar to 4,4,4, 15. The most important numbers in cas latency are the first 3. Make sure you run your ram modules at the right voltage. Sub timings can also make an impact.