800 watts is not actually very much heat. A typical small space heater is 2000W. I have no idea what you intend by implying that 800 watts is not as many BTUs as 600 watts--watts are literally a measure of power, i.e. can be converted to BTUs per hour. And yes, all those 800 watts of power your computer draws do end up as waste heat [1].

> I'm pretty sure it's going to be wasted watts too instead of using a real heater.

That makes no sense. You literally cannot "waste" waste heat, unless you are venting it to outside or something. If your space is not getting warm enough, it's because your computer is not generating enough heat!

[1] only a very miniscule amount of the power consumed by a PC actually permanently flips bits on harddisks or SSDs or transmits bits across the network. These take extremely low amounts of power.