Firstly, monitors don't speed up or slow down computers. You may notice a better quality of picture with CRT although modern TFTs are catching up fast. Though i might add i'm happy with my TFT.
With CRT you can also use higher resolutions like UXGA (1600 x 1200)..most (im sure there are exceptions to this) 17-19" TFTs only support up to SXGA2 (1280 x 1024)
Secondly, are you sure you have a "plasma" and not an "LCD"?...there is a difference.
Rob.

For FS9, I would think an LCD would be fine.

I fly a lot of online combat, and I require a CRT for diplaying the detail of very high closing speeds between fighter aircraft.

CRT's are better for fast action displays, hands down.

CRT's are cheaper, so you can buy a bigger screen than a comparitively priced LCD.

LCD's are lightweight, trendy, look great, and depending on type/quality, are pretty darn good these days.

If you don't require the fastest display possible, and are willing to pay a bit extra, go for the LCD, but get an 8ms one, as even at 8ms, fast action will blur slightly; the liquid crystals simply cannot refresh quickly enough with current technology.