CPU’s usage of RAM

Hello, first time here, I’ve got a question.

I run a 2.8 GHz Intel Pentium 4 CPU, hyper threaded. I currently have 1.5GB RAM.

My question is, i know there is a write speed on the processor that makes RAM above that ammount totally useless.

With a 2.8GHz processor, what is the maximum RAM it would use. I know my motherboard supports up to 4GB, but it seems to me that 2.8GHz would not be able to actually need 4GB of RAM.

Thanks a lot!
-Ian