The 7800 needs some little modifications to run call manager.
If you already pay Cisco for the software, you will be better off if you buy the 7800 from them, or they will not support you.
Now if you have not bought the software yet, you could, in theory,just get any server, do the small modifications yourself, get the software and try to make it work without support.
Cheaper but VERY dificult to do, unless you are already half genius.