no, the 8400gs doesnt meet the minimum requirements for those games and neither does your p4. Read the minimum requirements for the games you want to play. to play them well you must have ABOVE the minimum requirements. For example assassins creed has a minimum of 6800gt vid card and dual core processor.http://kotaku.com/347797/assassins-creed-pc-specs-your-pc-will-weep-like-a-scolded-childhttp://www.nvidia.com/page/geforce_6800.htmlhttp://www.nvidia.com/object/geforce_8400.html
The 6800gt has twice the fill rate and a HUGE amount more memory bandwidth than your 8400gs. So your vid card will not meet the minimum, nor will your CPU.