I use 2 pcs to do my rendering one is dual core 3GHz  with 3GB ram and win xp ,the other is Quad core 2.2 GHz with 4GB ram runing win vista from my experience a lot of the time when I was testing and giving them the same scene to render the dual core  would finish first though not by much and not always it seems to depend on the image though what factors i dont know. I dont know what difference (if any the graphics card makes) but my xp machine is my gaming machine and has a high end graphics card (gfx 260) In general i use both pcs together on the same scene using VUE render cow and that seems to be quicker over all so may be you could still use your old pc as well