resolution is 1200x960 and i like to play Skyrim and pes 2012.I also like to know maybe 128-bit memory interface card will be more suitable for me.
If there is little in it price wise, go for 1GB.
As for the games you play, if you would like to start enjoying the graphics of Skyrim you should probably raise your budget more inline with the 6670 series graphics card (price has dropped in the last 6 months for this little budget beater). This however takes away from your subject line I appreciate - and may not be an option but for the price difference this would be my starting point when looking to upgrade from the 8400. With your processor I suspect the 6450 will still be a bottleneck to your system and you could run a 6670 card comfortably without worrying about your CPU holding it back.