I'd be very appreciative of advice from photographers who've upgraded from the Nikon D300 to the D700. I've been using a D300 with a 17-55 and 70-200 2.8 Nikon lenses for theatre and dance photography, and have been frustrated with the noise and occasional lost shots that I've encountered while trying to use fast enough shutter speeds (1/200-1/320, f2.8) to capture fast-moving low-light indoor dance or theatre action between 800 to 1600 ISO. The 17-55 is fantastic for decently lit shots, but the 70-200 has delivered a lot more 'keepers' in low light work, on top of using NR software. I'm thinking about possibly upgrading to a D700, which I've heard is marvellous for low-light work up to 3600 ISO, but I've read that the 17-55 would most likely need to be replaced by the 24-70 lense, which I can't afford at the moment. Are there other similar 2.8 lenses that anyone can recommend that perform close to the 24-70 at slightly less cost? I've heard that Sigma lenses tend to be quite good, but haven't yet ventured away from Nikon glass. Any advice would be very much appreciated, many thanks in advance.