Memory Upgrade

[kingofgrills]

I am trying to upgrade the memory in my from 2 GB in order to improve its performance, and I could really use some help.

My Gateway GM5420 tower has 2 DDR 2 slots, with each currently holding a 1GB 240 pin DDR2 533MHz (PC2 4200) module (2GB total). My processor is a Intel Core 2 Duo 1.86. I am also running Windows Vista 32-bit, which uses a max of 4 GBs. The manual says I can install a max of 8 GBs (4GB in each slot). I assume there would be no real benefit for me to upgrade beyond 4 GBs, right?

My biggest question has to do with memory speed. My current memory runs at 533 MHz, and my data bus speed is 1066 MHz. Do I need to keep my new memory modules at 533 MHz, or can I use modules with a higher clock speed? Is it better to stick with 533?