If you open up your PC and take a look at the RAM, you should see some part numbers on the chips - you can Google these to try and find out the exact specification.
If you mix memory speeds, latencies, etc, your PC will use the slowest speed - you can run memory slower than spec, but not faster (unless you decide to overclock, of course). You need to look at the latency and so on, not just the bandwidth.
Does your motherboard support running unpaired memory? If it does and you have a 1GB stick - check the manual, or use Crucial's site to find out - try taking out a 1G DIMM to reduce the RAM to 3GB and see if the Windows index goes up. If it does, then Windows may be objecting to the fact that whilst you've got 4GB (good), it can't use it all (bad) - your only option here is to upgrade to 64-bit.
Either way, I personally wouldn't worry about a small decrease in Windows index scores. The scores do change as newer, faster hardware comes out.