Dear Plutonian Empire:
The Users Guide for Celestia version 1.3.2, which is available on the main Celestia site, contains a lot of info on which features in Celestia cause the biggest slowup problems. It also helps you understand what you can turn off.
Celestia has gotten very sophisticated. Its add-ons can add up quickly in memory usage. The base program generally can run on any modern machine, even without fancy graphics cards. It is the add-ons that start to drag things down. I know you've downloaded a lot of add-ons. UNfortunately, many of them do not have low res textures in them. For example, if you want to display some nebula add-on that you downloaded, there is typically only one texture in the folder ... a medium or high res texture. You cannot instruct Celestia to display only a low res texture for that add-on, because one has not been provided. It will load only what it can find.
You can convert some of your add-on textures to lower resolution by resizing them. For example, if a texture is 2046 x 1024 (known as a 2k texture), you can make it smaller using a free graphics program such as Irfanview. By converting it to 1024 x 768 (a 1k texture), you cut its memory demands dramatically. The add-on will still work, but the image will be a bit less sharp. However, Celestia will run much faster.
You can do the same with model meshes (3ds or CMOD), but you will need some software to do it with, and some reasonable knowledge of how to do it. I am not the one to ask, but others on the forum might be able to help.
Selden advised you to try loading less of your add-ons at one time. That is good advice. Celestia will load everything in your extras folder that is there. If you have 40 add-ons, it will load all 40. Not all of them will display at the same time, but enough of them will be drawn to freeze your system up solid.
To work that out, I have arranged my add-ons into separate folders, and have learned how to quickly change the celestia.cfg file to call up only certain folders that I want, and ignore lots of add-ons I don't want.
I have some good news. Chris is building into 1.4.0, a feature that will allow you to load only certain add-on folders, without the need to change the cfg file. This will help things greatly.
Until that is ready, I recommend you move out of your extras folder any add-on that you do not need to have loaded, and put it in a temporary extras folder. When Celestia launches, it will have only a limited number of add-ons to deal with. Also, convert add-on textures to lower res using a graphics editor, and read the Users Guide for helpful tips.
Hope that helps, and Regards,
PS - XRaider VIX. Lores textures is a legitimate question, but should not be a "sticky". Please remove the Sticky designation. Thanks