Alright now back to TS questions..
1. SB800 is a good buy, if u haven't got any speedlite..if budget is a constraint, SB600 will do fine..
2. The use of diffuser is to produce a soft uniform light..SB800 has a standard omnibounce alike diffuser supplied..for Sb600 u can purchase one ~$30 at CP..Most ppl uses diffuser to avoid the harsh lighting that may produce blown highlights when shooting direct..
3. Bouncing of flash off ceiling with or without bounce card is also a method for producing soft light..most often bouncing flash off ceiling with a bounce card is very sufficient..but have to take note of the ceiling height and color of the ceiling..cause bouncing off a colored painted ceiling(other than white) may get a color cast on ur pictures..
4. All flash comes with a guide number, including ur built-in flash
The formula is something like: Distance = GN/Fstop
GN given by manufacturer is usually rated at ISO100..hence if using a different ISO ur GN will oso have to multiply by a factor..Fstop is the aperture no. u r shooting at..these gives a good gauge on the distance ur flash can reach..
But with modern flash technology, like Nikon i-TTL and Canon E-TTL both working on the same principles of TTL..the formula above is almost obsolete for photogs..
TTL allows the flash to measure the flash power to fire by firing a pre-flash to measure the correct exposure for the subject..
5. like wat tribal.snake mention..SB800 can cover a range more than SB600..almost nearing to double..
Although flash lights are almost foolproof for users with current technology..but sometimes it might be fooled by ambient lightings..that is why flash compensation is build in..since being a backup photog..there is no harm in trying things..remember to review pictures after shooting and dial in compensation as needed..NOTE: Shoot RAW to play safe..so pictures can be saved..:bsmilie:
Just my 2Cent$..