Brand new to editing and would like help, please

After editing video on Adobe Premiere Elements 2, playback is very jerky and records jerkily when I transfer back to my digital camera.

Records ok on dvd, but I would like to record back to camera.

Computer Pentium 4 (3.06 GHz) 512 MB of RAM and 120 Hard Drive

Wd external hard drive help and, if so, how much would I need?

Hope you can help.

Patricia Ann