For my GTA IV playthru I am using these.
1. Better City Textures.....7.5GB and well worth it
2. Enb Series that enables FXAA
3. Some cars that I just picked out...not a car pack or anything like that.
The only thing with the enb series is that it can be way to bright indoors....you can tweak the config, but I never really got happy with it, but it is alittle better.
I am using a hard copy of the game, not a steam version...and I only patched the game to 1.04.....More mod compatibility with this patch and also the most mods were made for this patch.
The Better City Textures is a must and is just great work.
After that just find an enb that works for you, there are lots of them out there.
I am nearing completion of the game and it has been a great ride.
I was not sure about this game when I bought it, but I must say...overall I really enjoyed cruising Liberty City & listening to Liberty Rock Radio!
Looking forward to Episodes from Liberty City!