SB-800 as transmitter?

If i'm using a Nikon D3000 and I have a SB-800 (master) speed light and a SB-600 (slave) speed light, do i need to purchase set of triggers to set off the slave flash? Or will the SB-800 act as a transmitter?

No, that's it. Only limitation is that it's pretty much line of sight.

The SB800 will sit on the hotshoe of the D3000 and act as a Master & optically trigger the remote SB600 (it means you only have one flash 'off camera'). The SB800 HAS to be on the hotshoe to act as a Master flash. The Nikon CLS system (which is what their wireless flash system is called) allows TTL control of flash.

If you don't mind controlling flash output manually (it's not difficult), then a set of radio triggers (RF602's) and cheaper manual flashes will get you an off camera system at a lot less cost.

Suggested triggers = RF602's - £27 for one transmitter & one receiver, about £45 for a transmitter & 2 receivers (you can buy extra receivers anytime as you get extra flashes)

Suggest manual flashes = Yongnuo 460-II (£40) or YN560 (£60) - both fully manually adjustable in power output (YN46-II goes down to 1/64, the YN560 goes down to 1/128). The YN560 also has a zoom head (like the SB800).

By giving up TTL flash, you save a considerable amount of money, which you can put towards getting light stands & modifiers (you'll be needing a softbox or umbrella at some stage, especially if you're doing portraiture).