I've successfully upgraded 7 of my Dell computers with Core 2 Quad (Q6600) processors:

2 Optiplex 745 Desktops

1 Optiplex 745 Small Form Factor

2 Optiplex 755 Desktops

1 Optiplex 330 Desktop

1 Dimension E520

Hi..Im bought Intel Core 2 Extreme QX6800 2.93 GHz Quad-Core (HH80562PH0778M) Processor

Will this processor work with Optiplex 745 Small Form Factor?

Also which fastest memory I can install on this computer?..gfc Im buy Sapphire 11202-10-20G AMD Radeon HD 7750 1GB DDR5 Low Profile Graphics Card (HDMI, DVI-I, Mini Display Port, PCI Express 3.0, 128-Bit, AMD Eyefinity)

Oooohh.  so I have to sell this cpu...but before I sell I will try..whats happen....

what about Intel Core 2 Quad Q6700 - 2.66 GHz Quad-Core .. will be better as q6600?

On another forum somebody suggest me this memory  http://www.amazon.co.uk/G-Skill-1066MHz-PC8500-DDR2-Memory/dp/B0013K2YMI/ref=pd_sim_sbs_computers_2

will be working on SFF .. motherboard has 4slots for memory ...but I think only 2x2gb I can install if Im use ddr2-800

if Im use ddr2-667 I can instal 8gb of ram...so whats will be better..I want use computer for gaming on low setting.

Pretty sure that the qx6800 needs a bigger PSU than the q6600 I installed, and I am not sure anybody has successfully gotten the qx6800 into a sff computer without adjustments to the cooling and psu.  You probably will need a bigger psu since you have the small form factor and I believe the 745 desktop form factor has a bigger psu than the small form factor, especially since you want to power a radeon 7750 as well. 

As for memory, I believe you might be able to install ddr2-800, but ddr-667 should be fine.  I'm not sure it's going to make a practical difference.  You'll need a 64 bit OS to take advantage of more than 4 gb of ram in Windows.  Make sure it is ddr2, not ddr3.  I think that the SFF has 4 slots, but it might have 2. 

I think 4 x 2gb ddr2-800 will read at 8gb of memory, and should work in windows 64 bit as 8gb.  It might slow down to ddr2-667 speeds if you install 4 of them but I am not positive.  I don't think it will matter much for actual day to day performance what speed you have.  But I am not positive and only have 4 x 1gb in my Optiplex 745 at the time and can't remember what speed I had when I had 4 x 2gb (I think I had 4 x 2gb of ddr2-667 but have sold that memory off a while ago).

q6700 should work; I think the power needs are the same as the q6600.  It'll obviously be 266 mhz faster but I went with the q6600 since they were plentiful and cheaper at the time, and the SLACR version apparently runs very cool and needs little power.  I think the q6700 is the same spinning, so it should work. 

Let us know what happens with the qx6800 if you ever try to stick it in.

Ok..if not benefit..I will buy some cheaper 4x2gb

I don't know, but I don't see why it wouldn't.  That being said, if ddr2-667 or ddr2-800 is cheaper, you might be better with those.  I don't think you'll get any benefit with ddr2-1066.

will be this memory

1066MHz 

 http://www.amazon.co.uk/G-Skill-1066MHz-PC8500-DDR2-Memory/dp/B0013K2YMI/ref=pd_sim_sbs_computers_2 

 working on Dell SFF ?

It's not worth it on a 2006 system being 8 years old now.  The Core 2 Duo E6600 that you have is good enough.  

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Bev.