Mixing different RAM won't solve your problems, try using 2x2GB [U]only[/U], to see if that helps.
By relaxing your memory timings I mean increasing the latencies from current 5-5-5-18-2T to e.g. 6-6-6-20-2T... you'll have to experiment, the same applies to Vram setting (from default 1.8V to e.g. 1.9V) and also chipset / HTT voltages.
I don't have your motherboard so I cannot be more specific - you have to experiment a little bit.

just out of curiousity, which operating system are you using? from what little i understand, you need a 64 bit OS to use 8 gb. plain ol' windows vista and XP can't use that much, i don't think.

from first post
[quote]its 64 bit version of windows vista home premium.[/quote]