Here's a mod that's gaining some popularity in the PC community. It's called iCEnchancer (quite a few photos included in the link as well as news updates) and it was made to well "enchance" the visuals of Grand Theft Auto 4. It does the job clearly to full effect, but not without a few drawbacks as you will definitely need a gaming powerhouse rig in order to run this thing smoothly. When you can get it to run, however, it runs like a dream.
The current released version of it is 1.25, but he's nearly the completion of 1.3 which is supposed to fix a few things and increase the details even more. Make sure to follow the instructions in the readme file as it will show you how to install it and recommend the needed texture packs to reach it's maximum potiential.