Alright, for you Primarch's Curse, put down "The Flesh is Weak". Basically, once this comes into play, it'll mean that you start to hate organics more and more, and replace more and more of yourself with bionics, in an attempt to become a true machine. You will also beome more and more loyal towards the Omnissiah and Machine Gods and less loyal to the Emperor. God forbid you run into Necrons. (insert evil laugh here)
For your Chapter trapping...Let's say that you can choose a hammer icon, or a iron fist icon. Just like all the others, they ahve no real effect on your abilities, since this is an RP, not the full-fledged game. However, these are what help remind you of your chapter and where you came from. They are very important to you.
For you Chapter Demeanor, put "Remorseless Machine". Basically, this means that you, and your chapter as a whole, show absolutely NO sympathy or leniency towards heretics. For an example of this, take a look at what they did in retaking the Contqual Subsector.
For those of you still wondering what Insanity points and the Primarch's Curse are for, I shall explain with an analogy. Sadly only those familiar with the Star Wars d20 game will understand it.
In Star Wars d20, there are Dark Side points. As you do evil things, or choose not to stop evil things, you may accumulate Dark Side points. Once you reach a certain amount, your character is consumed by the Dark Side and turned into a psychotic DM-controlled NPC of epic evil-doom. Insanity points are like Dark Side points, and the Primarch's Curse is what determines how you act as you progress further along the path to Chaos. Insanity points will range from 0 to 99. At 100, you fall to the sweet whispers and false promises of chaos. Now, unlike Dark Side points, which can only be accumulated through actions you willingly do or do not take, Insanity Points can accumulate from simply being exposed to chaos taint. Chaos taint such as a member of the Traitor Legions, or a tiny, harmless warp anomaly, or a full blown Chaos Incursion. Also, should you become tainted enough to gain a mutation, no matter how slight, you will continuously gain Insanity Points, as the Chaos gods and their servants now have a direct link to you. Too bad you don't have an Apothecary to monitor your gene-seeds.
PS: Keep your PM Boxes spacious, as there will be ALOT of GM-Player communication. You know, since only Librains can see into the warp (without going insane...as easily...) and Techmarines are the only ones who can speak the language of the machine. And speaking of languages, in order to give Tac Marines more RP options, they can have one Xeno Language that they speak.