The 9400GT should be a bit quicker than your current IGP, but the difference should not be to extreme.
If you are gaming and performance is nearly playable, the 9400GT might bring it playable.
It will not, however, make any games that are not even close to playable perform fluidly.
With such a small increase in performance to be had, I would probably wait.
On the other hand though, it would only take about 15min to install the card; so if you need a bit of extra performance now there is no harm using it.