E6500 memory upgrade

Hi,

The E6500 is upgradable to 8GB RAM, I currently have 4GB in 2 SIMMS

Two questions:

1. Can I replace one of the 2GB SIMMS with a 4GB SIMM and get to 6GB?
2. Do you know why there isn't any way of purchasing more than 1GB SIMMS on the Dell website?

regards,

RR