Re: RAM question...
Your video memory is treated seperately from your system memory. Video memory is used purely for video processing, holding textures, rendering scenes, etc etc etc. Unless you're using your GPU for Cuda processing or whatnot, in games it's being used for video only.
Your system RAM, however, is where all the processes, and programs, that are running are held in memory. This includes your flight sim.
Now, it might appear that because you are using a higher spec'd scenary package that it's the video memory that matters - this might be the case for rendering each frame for throwing out to the monitor - but the scenery information and data, as well as all the processing and building of the world, is actually loaded from your hard drive and processed and stored in the system RAM - and it's this that the dev is talking about. The Graphics card is, in 90% of cases, only concerned about the rendering of the world.
If your machine is running quite lean (Not too many background processes, etc) then the 8GB of RAM that you currently have might be fine. I've a friend who lives and breathes flight sims, and some of the specs required for CPU and memory are pretty high - much higher than I ever thought, especially compared to the requirement for his GPU. They appear to be much more CPU intensive than typical games, and as such I would expect the CPU and memory requirements, in this case, to be much higher compared to other games.
Ultimately though, the only way to know for sure is to install the package and see how it runs. If it's borderline, then try and close anything running in the background and try again. If you're not happy, then by all means, ponder more about upgrading your memory.
Delta: "What's wrong Chris?? Chris: "I miss my old Cyrix"