HP compaq dc 7800 make NAS?

Yes, it is possible. Should work perfectly fine as long as you only want basic NAS functionality. Having only 2 GB may prevent you from doing more demanding and interesting stuff. But 2GB should be plenty for the basic stuff.

You might want to think of this NAS as a way to learn and get more experience. Later you may want to upgrade to something bigger and better or something smaller and more power efficient. And something that is powerful enough to run several dockers for streaming, downloads and other stuff.

I would recommend that you buy bigger HDDs. Bigger than you need now. Otherwise you are likely to want to upgrade soon. And good HDDs can last for many years. The warranty is 2-5 years and many HDDs last way longer than that. When/if you upgrade sometime in the future you can use these HDDs in the new NAS, making the future upgrade cheaper.

Or try to get some old used 1TB HDDs from someone that has upgraded to bigger HDDs. Buying a new 1TB drive today, when there are 18TB drives for sale, and 8TB SSDs, seems backwards. I suspect there are plenty of cheap used 1TB drives available, if you can find them.

It's a 64bit CPU, so that is a good start. I'd agree w/ the above the RAM limitation would be more frustrating than anything (for my use). Do you know how much RAM the board will recognize and how many slots it has? Looking here it has 2 SODIMM or 2-4 regular DIMM slots.

https://support.hp.com/us-en/p…459241/document/c01163881

Looking on Newegg at the RAM listed there, 4gigs (2x2) could be had for as little as $13 bucks. The bigger frustration would be the lack of drive bays.. but if you can make it work, I'd use it.