Given the equipment you have listed, I would say an entire kit replacement might be in order. The Nikon D70s looks like a decent DSLR, however it does not have the greatest high-ISO performance. Higher ISO capability with low noise would be a huge boon for photographing concerts (which tend to be quite dark.) Something that can handle ISO 1600, possibly even ISO 3200, without a lot of noise would be a LOT better for photographing dark scenes like concerts.
Neither of your lenses are very fast, with the fastest aperture being f/3.5 on the 18-70mm. I would highly recommend f/2.8 at at the very least, with f/1.8 or f/1.4 being far preferable. A 50mm, 35mm, or 24mm lens at f/1.8 at least, or f/1.4 if possible, would be fast enough for you to capture some decent concert shots. If you keep your existing camera body, I would say shoot for f/1.4 or f/1.2 if you can, as that should allow you to use ISO 400-800 (however, your shots are still going to be pretty dark.)
If you intend to photograph concert musicians up close, 24mm, 35mm, or 50mm lenses should suffice. If you think you might be photographing from farther away, or want to get closer shots with decent bokeh, 85mm, 100mm, or 135mm lenses would be better. Again, the fastest lens you can get would be ideal. At 85mm you should be able to get as wide as f/1.2, for 100mm anr 135mm you might be able to find lenses as fast as f/2, however f/2.8 should work.
Lenses with wider apertures tend to have thinner DOF, which might pose a small challenge for getting perfect focus. A camera body with better ISO performance, up to ISO 3200, would greatly help in this area, as you could stop down the lens to increase your DOF. On the flip side, thinner DOF results in smoother background blur (bokeh). Not sure that bokeh will really be an issue for concerts...backgrounds will probably be all dark or black anyway.