[inkista]

Nope, you don't need to buy a new camera. The current Nikon speedlights (SB-600 and SB-900) will work just fine. Older flashes or third party flashes will also work, but there are varying degrees of compatibility and function. For example, as zona5101 said, with an SB-24, you won't have any i-TTL automated power-setting, but the flash can still do its guide number calculation thing in Auto mode, and of course, the flash will still function perfectly well in full Manual mode.

The reason you might want to go with one of the expensive current SBs is because they have more advanced function and are made to work with digital SLRs, so they can do fancy tricks like letting you use shutter speeds higher than the D70's max. synch speed (1/500s), automated power-setting via through-the-lens (TTL) metering, remote power-control of off-camera flashes, and repeating mode. In addition, the SB-900 has full 360° swivel capability, which makes it great for bouncing flash with more directional control.

But if you just want to throw more light on something, and particularly if you're considering doing off-camera Strobist-style work with cheap radio triggers, then you could go with (considerably cheaper) older SB-24/26/28s, or even manual-only flashes, like the LumoPro LP120.