4gigs of 1600mhz or 8gigs of 1600mhz same ram Kingston (KHX1600C9D3K2/4GX) for both. Gaming system would it be worth the extra 4gigs or is there no benifit to having the extra 4gigs.

ASUS 890FX M4A89TD PRO
AMD PII X4 965 3.4
HD5870

Bigger is better. I got 8GB in this bad boy way back in 2008 when 8GB was actually impressive. With only 4GB you're going to find yourself hitting walls if you do a lot of graphical gee-wizzery.

I dunno about DDR3 mobos and needing matched sticks for better compatabilty. Next system I'm getting will have 16GB of ram, just cuz I can.

No need to get a new processor though, unless you absolutely must.

New MOBO CPU and MEM because the old CPU MEM AND MOBO is going to my son the 2.4 dual core AMD in his system is no longer enough to run new games like COD BLACKOPS and itsa waste of 70 bucks if it cant be played.

i didn't notice a difference in games going from 4GB to 8GB. However, i'm not doing eyefinity or anything like that and i'm only setting my resolution to 1680x1050. so maybe running a higher resolution or more monitors will make more of a difference assuming your video card or CPU don't hold you back first.