I would look at the 8600M GT or even better the 8700M GT. But the 8600M GS will be a big improvement over the 8400M GS. How about this one:
http://www.newegg.com/Product/Produc...82E16834220163
Asus wasn't nice enough to give you dual channel RAM, so I would pick up one of these if possible. If you can't afford it, get it at a later date.
http://www.newegg.com/Product/Produc...82E16820227241