While you will likely be able to make a FreeNAS server using your parts, as long as you are aware that your system doesn't meet the minimum specs for FreeNAS, then you have accepted the risks. Just keep a backup of your storage, well the important stuff because it could become corrupt. Yes, true story, bit flips and stuck bits do occur and more often than you would think.
All you need is a 64 bit CPU, the AMD will do fine, 8GB RAM or more, a NIC which you got, and some hard drives. Your system will not be a speed demon but it will function.
My advice is for you to run MemTest86 for 1 solid week uninterrupted. If you have no RAM errors then proceed. I say run it for 1 week because you don't have ECC RAM and this will at least ensure you have a stable system overall. Then run a CPU stress test of your choice for about 20 minutes. If the system survives then proceed to taking care of your data. If you are using old hard drives, run a SMART Long test on them and check the results. If that works out, I think you are ready to run the FreeNAS software and get that server online.
If you ever do need help in the future with this system, please ensure you list the hardware and of courser FreeNAS version. This isn't so people will tell you that you have an inferior system (they will do that, heck I just did that), but it may help us give you better advice.
You also stated in your power consumption that the video card was removed. I would not remove the card until after you have FreeNAS working perfectly + 1 month. You will eventually find something where you need a monitor.
Lastly, I would expect your power consumption to drop once you have FreeNAS running and the CPU can be placed in a lower power mode (Under System -> Advanced -> Power daemon), and there are some other tricks you might be able to take advantage of as well.
