it's mainly down to want u want to achieve with the flash.
Most importantly, SB-600 has a lower guide number compared to the SB-800.
BUT unless u need all the FANCIER features of the SB-800 such as CLS commander for MULTIPLE remote flash groups, Modelling Light, Stroboscopic Flash, higher GN, Auto Aperture mode, faster recycling via 5th battery or SD-8A ext batt pack etc and have the money to spare then by all means get the SB-800.
Most amateurs will nvr get more than 2 speedlights and it's more of a psychological want rather than a need to get the SB-800 for CLS. The only reason i can think of is to hook up multiple CLS flash during group model shoots when each photographer contributes his own Speedlight for group use. But how often do you go for such events?
Another important note is that if u own a non-CLS commander- capable camera body eg. D50 then a SB-800 is a credible addition for enhancing flash features.
"Good to have, but do u need a fancier Speedlight, Lens, Camera body etc to achieve what u shoot?"
Cheers