If you want to take advantage of more ram you'll need a 64 bit operating system. 2.5 GB of RAM seems like and odd number. I'm guessing you have more than that installed and then different hardware is eating pieces of the 4gb of available memory space? 1GB for the GPU, plus whatever else you have plugged in.
There's a couple of things limiting you in terms of comparing yourself to other Crysis benches. Your CPU is @ 2.8, which isn't bad, but most people posting benches are most likely 3+. And may not even be on a PII. The other thing is that the PII 820 only has 4mb of l3 cache. Better than the Althon quads, but prolly shaves a frame or 2 off when compared to the PIIs with a 6 meg cache.
Plus what you've already said, you have slow RAM.