Arc is, arguably, simpler than either Scheme or Common Lisp. The only reason for learning these other languages first, in my opinion, would be because of the greater availability of teaching materials.
In fact, if there's any language I'd recommend learning before arc it would be Haskell. You'll enjoy arc much more if you have a good understanding of functional programming first. The best way to do that (although a pretty hardcore way) is to learn Haskell.
There are two great problems when learning Arc: the lack of documentation and the quite cryptic error messages. For these two reasons I would reccomend to know a little lisp before learning Arc. A really good book is Practical Common Lisp (http://www.gigamonkeys.com/book/). I suggest reading at least the first ten chapters.
If you want to learn Scheme first try How to Design Programs or Structure and Interpretation of Computer Programs (both free online). HtDP is more beginner oriented, so if you can program I recommend SICP. There are valuable lessons in HtDP even if you are a programmer, but it's not as challenging as SICP so HtDP might be boring.
You don't need to know either (or any other language for that matter) to bang out a project of some kind via Arc.
But to hang with the language theory discussions here you are going to want to know everything you can about functional programming, and especially PLT's implementation of scheme, because that is what Arc runs on.
I want to learn Arc, but I'm afraid that it being unfinished will make all my knowledge obsolete when a new release comes out. Is that really a problem?
