the computer you have now will be ok for HD/1080p projects, but if you are attempting 4k it might need HD proxies to lower the demand on the current computer. premiere is mostly cpu bound, so the cpu would be the primary suspect for performance issues. i doubt a new gtx video card will help much unless you are doing a large amount of gpu accelerated fx. you can check the gpu usage (load %) with gpu-z to see if the video card is overworked vs the cpu usage in windows task manager. if the cpu usage is at or near max when the jerky playback happens the cpu might be causing or contributing to the jerky playback. if you are working with very high bitrate media, and/or multiple layers of media in the timeline, it could be possible a ssd could help. you can use windows resource monitor to watch the media hdd and see how active it is, that will give some clue to the hdd speed being an issue. you could try moving some of the media to the ssd and testing it in a new project, that would help determine if the hdd is too slow.
as far as new computers go, intel has stagnated in cpu performance, so a new 4 core would be only slightly faster than the one you have now. to get a faster computer it will need more cpu cores. a workstation from hp or desktop pc from a custom builder with a 6-core to 8-core cpu will be between $3k-5k. the performance difference could be big, the 8 core cpu option's being around 2x faster.
Thanks for your input, that's much appreciated. Did the GPU-Z test and video card seemed to check out ok, so it would appear your assessment may be correct. Thanks again.