Christianity is a monotheistic religion, even if that sometimes makes the Christians uncomfortable.
Not only is it a monotheistic religion, but it posits that God is allpowerful.
For God you need to set his divine rank at say 30 or something, essentially high enough, that for all practical purposes it is all powerful. God has access to all domains, which means essentially that he can cast any spell at will. However, God does not exercise his right over certain of those domains. He considers them beneath his Holy self, and he does not grant spells from those domains and only rarely uses them (to destroy something he considers abominable), and in fact gets somewhat miffed when people ask for them as if they had a right to be offended about what someone else had done considering the sorts of things they themselves had done.
God occassionally seems to act through agents who manifest a certain level of divine power, but noone but God grants spells, so no one but has Divine rank 1 or higher. However, just because no one else has divine ranks doesn't mean that some of the other actors involved in the story aren't sufficiently powerful that humans wouldn't get confused over such a fine detail. It is quite reasonable to assume for instance that Satan has 60 or 80 or 100 (effective) character levels, and therefore Epic enough that the distinction between him and 'a god' may seem overly fine to the non-theologically minded.
It should be noted that god doesn't grant _spells_, only miracles. Christian clerics don't recieve spells. As a consequence they have much less control over what happens (spells never get used for purposes God isn't interested in, ei the DM always fiats any spell use he believes inappropriate), but on the other hand they don't have to select spells and can expect whatever miracles that thier level of faith allows. It should be noted that attempts to cast an inappropriate spell not only fail, but still use up the spell for the day (since they generally try the faith of the cleric).
As for Domains, clerics of God (under whatever name), have free access to any of the following domains: Good, Protection, Healing, Sun, Knowledge, Creation, Nobility, Renewal, Family, and so forth.
As well, they may take up to one of the following restricted domains (which God considers to be somewhat though not entirely missing the point): War, Strength, Law, Chaos, Animal, Plant, Earth, Fire, Water, Air, Storm, Luck, and so forth.
Domains like Destruction, Death, and so forth God reserves to himself since he doubts anyone else has the wisdom to use them properly anyway. The Evil domain God has no real reason or desire to indulge in. It is these reserved domains that the enemies of God seem most interested in.
I would assume that all the named spiritual beings who are not God have Divine Rank 0. These beings grant spells by direct petition, which is to say, they cast the spells not the enactor (which actually is true of god as well) when they notice someone asking them to interveen. It particularly amuses the evil enactors to make the 'spell caster' think that the power derives from themselves or that somehow they have forced the enactor to act this way. Still, from a cleric of Satan's (who may or may not believe that he is a cleric of Satan) perspective, that's just exactly what's happening, so from a (N)PC's perspective the usual D&D mechanic works just fine. However, the higher level the spell in question, the more likely it is that Satan is far too busy to spend his limited power granting it. Satan has the broadest access to spells of the domains of trickery and evil, and you may assume his clerics have these domains.
Interestingly, Christianity has never claimed that 'arcane' magic is impossible, and there is even a hint that it is specifically forbid because it is possible - and horribly dangerous. Also, God considers it missing the point, because its about the acquisition of personal power and any personal power other than his own looks pretty cheap to him.
It would not be a bad idea at all to visit the Middle Earth D20 site and read the comments there regarding magic use, since they would apply equally well to a fantastic magic Earth.
Last point, I don't really recommend you running a campaign centered around God and Satan. It seems to me, if you'll pardon the preaching, just a little more dangerous than Thor, Zeus, Baccob, and the like. It also potentially cuts too close to home for some people, especially people who enjoy being offended.
