If you want to use the lenses you currently have (D50 is DX format) then the D7000 is the way to go as it is also a DX format camera. If you are going to buy a new lens or lenses then you could go either way. The FX lenses for a D700 are going to be more expensive then the DX lenses, but there are advantages to FX sensors and lenses. Do more research, but either camera is an amazing choice and will do what you want WAAAAAAAAAY better then the D50!
There are some great video's on youtube explaining the differences between Full Frame (FX) and Crop Sensors (DX)
