It honestly depends on the situation, a great number to shoot for is 150 before buffs bard songs and potions. However, there are some dragons that have a negative 150 modifier to their AoEs, so you really have to work to resist them.
Most often Magic is the most important resist for things like: fear, snare, root, gravity flux, certain AoEs. In several situations for dragons you will want to make sure you resist the fear before resisting the AoE, a feared healer is not healing.
Poison and Disease are the less important resists, but still critical in certain circumstances (Trakanon (PR), Severilious (PR), Hoshkar (DR), other shaman based mobs). Poison is easy to stack with DMF, Resist Poison, and Shadoo stacking in addition to Blue Diamond Earrings and Platinum Pearl rings.
Fire and Cold are more important, but still not as much as Magic. Depending on the class/race these can be easy or hard to stack deeply. Stuff like Nagafen, Talendor, Faydedar, Gorenaire, and Vox you want Magic as a priority over Fire/Cold to avoid the fear, but still enough Fire/Cold to resist. Cold helps in dungeons if you fight a lot of wizard mobs that like to drop Ice Comets on your head.
A good resource for figuring out resist gear sets is the wiki. From the Equipment by Class page, select your desired class, then the specific slot you want to examine and sort it for each resist type to find the different pieces that work for your class/race/budget.
http://wiki.project1999.org/index.ph...pment_By_Class
edit: typo*