(Update - 20th June)
To save people having to read through the whole topic, here's a quick summary of the most common issues and some potential solutions:
1) If your performance has gotten noticeably worse, go to the Game menu and ensure that Clip Capture
is turned off.
2) If you get errors when installing the patch, make sure that GTA IV and Social Club aren't running.
3) Delete any mods if the game won't start. Remove any ASI files. As a last resort, try re-installing the game.
4) ATI and NVIDIA released new drivers this week, so try updating those: http://www.gta4.net/...-ati-and-nvidia
5) If the game is stuttering, read this: http://www.gtaforums...howtopic=404147
6) Refer to the modding forum
if your mods no longer work. Things like ScriptHook
Don't use the benchmark tool to determine your game's performance, because it isn't entirely accurate. I got an extra 5-6 fps after installing the new patch, but the benchmark tool indicates otherwise. Play the game and judge for yourself.
Edited by adamcs, 20 June 2009 - 09:29 PM.