Hey SaVas,
Sorry for the late reply but they don't let me out in public much anymore.
After reading the comments from the other PSP users, I'd agree that 7 is the best version to start with.
However, I've really grown to love one feature in 8 (8.1 actually, because like the others said, 8 was buggy)...when you get comfortable with layers and start to build them at an alarming rate, you'll understand why the new feature in 8 called Group Layers is so great.
Without getting into a ton of boring detail about the many things I use it for, let's just say that I wouldn't paint half as fast as I do now without it.
Of course, I've been doing this for over a year now, and painting with a freeware group, so I tend to do this alot more than others.
Here is a few 737s I'm planning for the new Kittyhawk 737 series coming in the future.
There are 737-300's, 400s and 500s all mixed in there, and they were all painted with PSP8.1. (sorry for the poor resolution of the picture...heavy jpeg compression did her in)
I've painted well over 100 planes, and people tell me the quality is pretty good for the quantity I produce, so I have to say that PSP, dispite what some Photoshop purists will lead you to believe, can handle painting planes for flightsim just fine.