I had this same problem a few years ago when I switched from film to digital. I opted for the sb-600 and it has been everything I have wanted and needed until now. Now I am using the flashgun off camera, with softboxes and umbrellas which loose you valuable stops of light, and at times find myself wanting a little extra power which the sb-800 would give me.
To be honest both flash units are excellent and it's hard to beat the value of the sb-600 which can now be bought online for around £124.