Upgrading NVIDIA nForce 680i

At first glance, it looks like you need to up the voltage on the ram.
CPU-Z says they are at 1.8v, but the specs for the OCZ says 2.1 - 2.3v. And even thought you are not running them at the rated 1066mhz, you still need to get that ram to at least 2.0v to run at 800mhz.

If that Winbond motherboard will not let you do that, you will need to get different ram with a lower voltage requirement.

what i can do to upgrade ram to 8 GB and what is better Ram spec and brand

This is Crucial's recommendation for your EVGA 122-CK-NF68 motherboard: http://www.crucial.com/store/listparts.aspx?model=nForce%20680i%20SLI%20(122-CK-NF68-AR)&Cat=RAM

1.8v is the correct voltage for the RAM.  You can get a pair of DDR2-667 4GB modules, but they are very expensive ($170 at newegg http://www.newegg.com/Product/Product.aspx?Item=N82E16820148274).  Your current configuration of 4GB total is good.

What exactly is wrong, that you want to upgrade?  The cpu, an E6600, can be upgraded to a quad core Q6600, but are you cpu limited?

when running Oracle E-business Suite on Linux VMware the Performance go down and ram appear as full. that is why would like to upgrade the PC.

your recommendation to get pair DDR2-667 4GB is not optimum solution because i think current ram is better.

if the ram upgraded to 8 Gb may be better, what's your opinion for two kits of
http://www.newegg.com/Product/Product.aspx?Item=N82E16820104073
and install Win 7 64 bit

as i understood i should replace current memory with new kits 2*4 DDR2-533. is it correct?
and 4*2 is not recommended .

You have understood correctly.