I wnt to upgrade my cpu for Gaming with existing motherboard ...

[vnegi2020]

MOBO: GA-M61PME-S2P rev 1.0

Existing CPU: AMD Athlon(tm) Processor LE-1640, ~2.7GHz

PSU: 300W

According to your website information about my MotherBoard is AM3 ready so which processor will give me highest speed without any
downgrade of  system bus speed  from HT3.0 to HT1.

[Dark Mantis]

Hi

There is a list of all the processors that your motherboard will support and if necessary any BIOS updates. Don't forget to flash your BIOS with any update before you replace your CPU.

http://uk.gigabyte.com/support-downloads/cpu-support-popup.aspx?pid=3009

Also your PSU of only 300W is too small anmd you should have a 550W minimum PSU.