XPS 410 Processor & RAM Upgrade

I have a Dell XPS 410 with Core2Duo 2.13 GHz, 2x2 GB Ram, Win7 Pro 64bit. BIOS 2.5.3 Updated (Dec2007). I have following questions,

1) After reviewing some conversations in the forum, I tried to increased my Ram from 4GB to 6GB. But after installing 3 DIMMs (2GB each), the windows properties shows this:

Physical Memory 6 GB (3GB Available).

It does not make any sense to me. Does it mean that I am still using less than 4GB?

2) All my RAMs are 800MHz. But including BIOS and other places it shows all rams to be 667MHz. Why is it so? How to make them working as 800MHz?

3) I see the list of upgradable processors HERE. the list does not include QX6800 core2quad processor. But QX6800 has same FSB speed and same 65nm. Can I use QX6800 to upgrade my pc?

Would you please help me to find the answer of these????

Thanks in advance.