Hello Bharat!
Could you let me know your motherboard model number?
#1 - Remove all the memory modules and install the 800MHz modules one at a time.
#2 - If it boots then try installing another module and check if it works.
#3 - If the new memory works alone but the memories do not work together then a BIOS upgrade should fix the problem.
If you see any error messages or hear any beep sounds then make a note of it. Each number beep denotes a certain error. If one new memory works (i.e 800MHz) but the other memory doesn't then the memory might be defective.
Also you can run a memory test -
memtest86 (Click here!!)
Let me know how it goes or if you need any more info.
Regards.