They are NOT FULLY compatible!
Your motherboard (MS-7318) uses VIA PT890 series chipset which supports maximum FSB of 1066MHz.
Core 2 Duo E8400 is the second generation of "Core 2" family. It has clock speed of 3.0GHz and FSB of 1333MHz so it runs at the multiplier of 9x.
There are two possible problems:
1) Your computer won't boot at all because your motherboard BIOS is very old so it may not recognize your new CPU.
2) Your computer will boot but your new CPU will run at lower clock speed of 2.4GHz.
The first problem can be solved if there is BIOS update for your motherboard which you have to get from your computer vendor.
But when it recognizes your new CPU, you will experience the second problem because your motherboard cannot handle FSB over 1066MHz.
So there are two suggestions for now:
1) If you can't solve the first problem then the only compatible CPUs for your motherboards are Core 2 Duo E6x00 series (e.g. this and this).
2) If you updated your BIOS and it can support this newer generation then you can get Core 2 Duo E7x00 (e.g. this and this) or Pentium Dual-Core E6x00 (e.g. this and this).
With some more research, it looks like VIA PT890 series chipset won't accept any 45nm Wolfdale-based CPUs at all, including Core 2 Duo E8x00 & E7x00 and Pentium Dual-Core E6x00 & E5x00. See my post here for more info.
^ Thank you!
Took me a while to research, though. Never heard of MS-7318 but later found out that it came from Medion PC MD 8822.
Also found that new BIOS update is available so I'm updating my previous post now...