I had the same dillema several months ago. I finally bought the SB-600, and I think I made the right decision.
Here are the main differences between them:
- The SB-800 can function as a commander unit that can control other flashes. This is relevant only if - a) you're planning to have more than a single flash and shoot with all flashes together (as in a studio setting); b) your camera is NOT the Nikon D70 or a more advanced model, which already has a commander unit in its built-in flash.
- The SB-800 has a higher guide number (i.e. it's stronger and the light reaches a greater distance). It's also somewhat heavier and larger than the SB600.
- The SB800 has a built-in bounce card - a small plastic that sits on top of the flash and reflects the light, thus producing a softer illumination than direct flash light. The card is tiny, though, and its effectiveness has been questioned. I plan to make my own card from a piece of white plastic and attach it with velcro to the flash head.
All other differences can be found here:
http://www.kenrockwell.com/nikon/sb600vs800.htm
For me, none of these differences warranted spending almost double the amount for the SB800. I figured that if I really want to get into multiple-flash photography I can always buy the SB800 later, and use it (as master) to control the SB600 (as a slave unit).
Whatever you choose, you'll be thrilled with the results. I find myself using the flash in many occasions, and my indoor photos look great, with a soft, natural look. BTW, whenever possible I tilt the flash head and bounce the light from the wall or ceiling. The result is a soft balanced lighting that resembels soft daylight.
Go get the flash (any of them) - you'll love it! 
