|Hybrid SLi is definitely better than onboard graphics alone.|
A 9000 series card would most likely perform better, but it would depend on the card & the Hybrid configuration. A 9600GT would kick the crap out of the Hybrid setup, but there may be very little difference between the Hybrid & a 9400GT. Again, it would depend on whether the 9400GT is a 64-bit or 128-bit card & which card is used for the Hybrid.
That being said, if you want good gaming performance, there's no substitute for a decent card. If you prefer nvidia cards, you may wanna look into the Geforce GT 200 series, but stay away from the lowly 210 & 220. Go for a 240 or better.
Check this article:
Best Graphics Cards For The Money: January 2010