first play it in its vanilla form, if you are not impressed with the graphics, go for mods & mods means you cant play online mp.
theres this ice mod for low end pc -
GTA4-Mods.com - Grand Theft Auto 4 car mods, tools, and more!
go thru comments carefully, its for patch 1.0.4 only.
while using ice, remember one more thing, GTA missions=scripted, so theres a lot of chance that you may never be able to complete some missions in which you have to chase cars/bikes..., since your pc will respond slower(lag).
btw, my core i5 2430m & gt540m run gta4/eflc silky smooth, with shadows on @1366x768.