Sorry, its a Dell Optiplex 745. Main-board model: 0RF703. With Intel Southbridge.
Ive got a C2D E6700 in it now and I want to put in a C2D E8400.
I don't believe it will work. LGA 775 had several generations. The first generation was the Pentium 4 5xx and 6xx and Pentium D chips. The second generation was the first generaton 65 nm Core2 chips like you have now. The last generation were the 45nm Core2 chips like the E8400 your friend gave you. While the older chips were supported on the newest motherboards the newer chips would not work on the older boards. This is especially true with the OEMs like Dell as they have no reason to make their systems upgradeable because they want you to buy a new Dell.