I dont think you need anything, the DVD rom in addition to your plextor would be a nice setup. You could get the NEC burner instead of the dvdrom,or use your current dvd recorder, and use it in place of the dvdrom. Since you have the plextor dvdr already, you have the best.
Certain drives perform better for certain tasks, et al safe disk2/psx/dae/dvd etc.
I would use my plex/existing drive for a while, and if i didnt like the setup, re-evaluate. I dont think you will need to change, unless one of them is weak in dvd ripping.