Different Frequency Of Ram

i have 2 different frequency of ram
1 gb transcend Ram ddr2 800mhz
1 gb dynet ram ddr 2 667mhz
and i am using gigabyte GA G31m ES2L

if i use both the ram in my motherboard can i have any problem in the future
like overheating,ram failure,etc

No problems, Just that both the rams will run at 667mhz.

1 gb transcend Ram ddr2 800mhz
will also run at 667 mhz

yes, if the other stick,, Dynet is 667 then both will run at 667. You wont have any problems.