nvidia ge force 8400gs graphic card problem

hi there, i recently installed nvidia geforce 8400gs on my desktop. my desktop has core2duo 2.2 ghz processor, 1 gb ram and has on board video card of128 mb memory. before i installed the graphic card my 3d modelling software ran smoothle unless the drawing file became too big. but after i hav installed the graphic card even the small files also feel terribly sluggish on my system.
reason i brought the graphic card was to improve real time on screen graphics display. but the installation has just resulted in the opppsite. my mother baord is asus x series. p5gc-mx . please help!!
i use google's sketchup for my 3d modelling needs which used to run fine on on bboard graphic capability till i installed this card. pls help. thanks