Why don't you try it and see what it sounds like ?
In other words - if you're using a software eq in a computer editor then generally you won't get artifacts for 1,2,3,4,or 5 dB boosts or so at 1/3, 1/2 or 1 octave bandwidths. If you're not using a parametric eq that has an adjustable bandwidth - like a graphic eq then you can 'feather' the sliders up a dB or 2 on each side of the 800Hz slider so they're about half as high on each side.
Depending on how steep you make the eq boost you might hear some artifacts but not till you get past 5,6,or 7 dB. If you're in a full
mix pre-mastering something and you find youself boosting 800Hz more than a few dB you probably have a bad mix so now you can say you're doing some serious re-balancing!
If you're on a single track boosting 800Hz then shape it however you like to fit in the mix. If the boost (gain) gets too much and too steep (slope, affected by Q) though you may hear some artifacts depending on the quality of EQ you have.
If you're using hardware then we'll have to wait for someone else to come along - that's where the analog phase stuff can happen. I don't really have any analog eq's.
Whatcha boostin and why ?
kylen