It sounds as though the .mdl or model file for some of your aircraft has been updated and the textures are for an older version.
This happened to me. Either the whole aircraft was grey or only the tail showed up in colour and the rest was grey. If you download a texture file for PAI
then, you have to make sure it is the correct version for the aircraft you are putting it on.
Some aircraft come with different variants. ie 747-400PW, 747-400GE, 747-400RR. If you put the wrong texture on the the wrong variant it also will not show up properly.
Here is something to look for:
747-200 is like this.
The texture files for my 747-200GE variant are "PAI
The texture files for my 747-200PW variant are "pai_742pw_t"
The texture files for my 747-200RR variant are "pai_742rr_t"
If you put say the RR
textures on the variant with GE
engines, then too you will get grey or better said, NO textures.
It has nothing to do with your graphics card or memory. It is just like trying to add Posky textures to the default aircraft.