Please experts, HELP OC!

hi.
i have:
1. core 2 duo E6750, 2.66 GHZ
2. case: THERMALTAKE SOPRANO WINDOW + 2x120MM FAN , 1x90MM FAN.
3. memory: corsair (2x1gb) XMS2 CM2X1024-6400C4 800MHZ

my questions are:

1. why is it that in everest it sais (under OC tab): cpu clock : "2000 MHZ (original: 2667 MHZ)"? why isn't it working at full speed?

2. i bought 2 new memory sticks: CORSAIR XMS2 (2x1GB) DDR2 1066MHZ PC2-8500 EPP SLI RAM.
now, i want to add them in addition to my old memory (800MHZ) but as i heard the motherboard can only use the slower MHZ (800) despite the fact that my new memory is 1066MHZ...
i tried OC through bios and then in "CPU-ID" software, under "DRAM frequency", the sped changed from 400 to 533.3 MHZ (which later on caused system crashes.... maybe because i didnt add voltage? i dont know...)

so what do u thing about my 2 issues?
thanks in advance!

1. Your motherboard probably has a setting enabled to allow the processor to clock back when it's not being used. It's a common feature in laptops (Right now my 2GHz processor is running at 1GHz to save power). You can disable that through the BIOS (which is also where you want to OC from).

2. Did you look up the specs of your motherboard before you purchased the RAM? If it can only support 800MHz, then that's all it'll support. Also, when you OC, it is best to go in 5MHz increments, stress testing inbetween to determine stability. You can't just up the speed by 133MHz and expect it to work. Remember that when you increase the FSB speed, it effects everything, RAM, CPU, Northbridge, everything.

What motherboard do you own?