Wich ddr2 Ram speed is compatible with Pentium 4 650 800Mhz ?

Both rams should work so it might be something else. HDD? GPU? PSU?

here's a list of tested ram for the MB. Other ram may work, but these have been tested.

http://www.asrock.com/mb/memory/G41C-GS.pdf

You need to look up the FSB speed of the Cpu. The cpu will support by the look of it the chip will support up to DD2 800Mhz memory. DDR2 6400.
Some boards cannot deal with mixed memory speeds as a rule both sticks should be of the same speed.
If you have a lower memory speed, then set the Memory in the bios manualy to the lowest memory speed it may work then.
Ie : FSB 533 memory speed at 266Mhz. Double data rate memory works like this 266Mhz up, 266Mhz down= 533mhz cpu FSB.

So cpu FSB set to 533Mhz and the memory speed set to 266Mhz

First of all thanks for your inputs.

1- I cant get to bios menu.
2- I double checked the MB Manual and in fact 533Mhz RAM wont work int this MB with 800Mhz CPU FSB.
3- Tried 2 Dimms DDR2 Corsair CM2X1024-5400C4 675Mhz, and they didn't work, although according to MB manual they could.
4- I didn't mix memory speeds.

Supposedly CPU should work with "the specified core to bus frequency ratio and lower.

Special Thanks to ss202sl will see if I can get some of those RAM.