I would like to have a illusionist/thief, but I would also like to get some really cool high level spells, and as many as possible. I know that multiclasssing will delay this, but will it stop it all together? Will I be able to get the high level spells (7,8,9)?

If I change the XP cap, will that make a difference?

Also, could I have a Archer/cleric multiclassed? Or a dual classed Archer/cleric for that matter? I want an archer in my team, but I also need the other classes like mage, cleric, thief, and some fighters.

Well, I can't really verify the dealio with the archer/cleric mix, but I did make a Ranger/Cleric mix. The only major problem with dual or multi-class characters will be the level cap that BGII has set. You can find an XP cap remover on some web-sites. Unless you do this, 5th or 6th level spells are gonna be the highest you can cast as a Cleric. This will go pretty much the same with the illusionist/thief mix too. The thief level will raise pretty quick, but the illusionist will go much slower. I believe 7th level illusionist spells will be the highest this character will get (again, with the xp cap not changed). If you decide to mix a fighter subclass with your cleric, remember that you have to do the subclass first. You can only choose a generic class with your dual class. BOTH classes count towards the XP.

the archer/cleric things shouldn't work if you want a archer as bow and arrows are against the clerics ethos. can someone thats done this confirm this as i havn't actually done it, but it's against what the manual states. with the thief combos generally if you don't go above level 9 (*4 backstab) 110,000 exp points you should be able to get to your highest level in the dual class. just check the tables before you do the change.

good gaming

See, like, see, like that's why I was saying it's better to start with cleric and then switch to a fighter. They don't get the benefits of the Ranger class, but they can use all the weapons a fighter can use and still cast clerical spells. Plus they can specialize. You want a good Archer, just specialize in Bow.