If it is the M4N72-E than with the latest BIOS you will certainly have a very decent gaming rig, plus it'll do all the encoding and media serving very well.
It's an above average rig if you were to ask me...... the setback is only the DDR2 but otherwise it's a very very decent rig....and yes if you have DDR2, the Phenom II x4 and the Mobo just lying around, then it's certainly worth putting together and gaming on.....
If you've got the M4N72-E then it's worth it! Make sure you update to the latest BIOS before changing the CPU. Also, as long as you have DDR2-800 or higher, your DDR2 won't hurt you that much in performance.
With the latest BIOS, you can install a Phenom II x6 on the M4N72-E if you wanted to, that board will support CPUs up to 140 watts.
Your title said M4N E... hence the confusion. Yes, it is still worth upgrading to the x4 955 or 965 on the M2N-E... they are supported on your board. But be sure to check the fine print for the CPU you want.
The Phenom II x4 955/965 have this note: "Due to Bios ROM size limitation remove LAN PXE ROM and change Crashfree3 to Crashfree"
Also you want to update to the latest BIOS before changing the CPU.
Changing the CPU on that board can be an investment, because if you want to change the board later in a year or 2, your CPU will still be pretty good. And Black Edition x4's can be overclocked quite well with a simple multiplier bump.