Yes, your RAM utilization is being hampered by your 32 bit operating system. Especially if your playing any of your games in directx 9.
Upgrading to a 64 bit OS would be a wise choice.
Keep in mind the 550 ti is a mainstream graphics card and has its limits as well.
Check this out for more info on why your RAM utilization is being murdered by your OS. http://www.techpowerup.com/forums/showthread.php?t=91260