According to the forum posts in the link below, the vostro will only support Core2Duo's. The E8400 worked for somebody, and I am looking to go to 3GHz-2.8 or so. I am looking to improve performance while gaming, and I have already upgraded the GPU to a 6670.
Unfortunately having the slimline edition limits your gaming ability due to the low profile requirement on the GPU. The 6670 is on the bottom end for gaming cards and doesnt even need power from the power supply.
yes an e8400/8500/8600 will work in your pc but how much better you will game is marginal. Most game require better GPU and not CPU.