The t7350 would be a downgrade, not an upgrade.
The p8400 would be a good upgrade, but it is impossible to know if it will work with your computer except by trial and error. Even if the chipset supports it, the bios might not. Even if the bios does, the laptop might not be able to handle the extra heat.
However, because the laptop originally could ship with up to a t9500, it is very likely that you could upgrade to the p8400 with no problems:http://www1.ap.dell.com/in/en/business/latit_performance/latit_d830/pd.aspx?refid=latit_d830&cs=inbsd1&s=bsd