I have recently bought a logtitek mic/camera and it works fine with skype on my workstation, however when I use it with my Dell D600 the sound quality is extremly poor on skype. When I play a CD it's ok and windows start-up sounds are ok.

i know they're both old though monies tight and the 510 works great, just has only 80g, the 620 has 250g. also, can i use the ram to increase the 620. the 620 comes with 2g ram and i upgraded my 510 years ago to 2.5g, can i remove and install in the 620.

We are not sure about the exact cause of the issue. I suggest that you try removing few hardware components one by one and then try starting the system to isolate the issue. You can try removing Hard drive, Optical drive, Memory sticks (One at a time), expansion cards (Graphic, sound etc.) if any.