Yasha, here's a brief wiki on achieving 100% status.

IGN GTA 4 Wiki

There's beaucoup mini-quests, challenges, etc you'll have to complete if you want to achieve 100% completion. Rumor has it that you can use the internet within the game to help you locate a lot of the side quests, etc.

In other news, according to wikipedia there's a glimmer of hope for PS3 owners regarding downloadable GTA 4 expansions. They may actually have it for PS3 too as it turns out.