When your computor and Gta start to run slow and crash, you have overdone it. I suggest take it easy on the mods. Many problems can possibly come up with many mods. Try not to overdo it or the game could possibly stop working or you might have to reinstall the game.
I would avoid ENB and ELS and graphic mods can not only slow down the game but your PC.
I would suggest looking around Gta 4 Mods main website for Graphics Enhancers and always take a close look at the reviews by other users before actually downloading a mod. Also make sure that you back up all of your files before installing mods and do not use your trainer during career mode. Also constantly save and do not save in the same slot. This means that if something happens you can easilly correct it and set it back to how it was originally.