Okay, so after posting i finished up reading the thread. So . . . ignore everything I've said. Yeah, frankly I think he;s a moron, or if not, he's as sarcastic as the rest of us . .. and since it's clear I don't need to be nice to try and explain it to him, I'm going to take this opportunity to have a good laugh at the sheer stupidity of all of this. If you go on reading after this, I think I sound nicer, but it should probably all be ignored.

Breadboy: but as I learn both hands seperately, perfect them at 120-150 percent of the final speed, then combine them, it shouldn't be that much of a problem until i have to put them together, and by then i should be able to do that
Don't count on it.
2 against 3 is the most common kind of . . . rhythm juxtaposition, and it's very easy compared to 3 against 4 -- or really, as in the case of the Fantasie-Impromptu, 6 against 8. What tends to happen with this piece is that the left hand becomes uneven and starts sounding a little like iambic pentameter (or, I guess, sextameter). Unless you get the rhythm ABSOLUTELY solid from the get-go, it's not going to come out right when you play it fast, and since it is entirely possible to listen to either the right hand or left hand melody (I learned this by recording myself playing it), if either one is uneven or doesn't fit correctly, it's going to be obvious.
I suppose a lot of people recommend learning it the way you're planning to . . . but in my experience, it takes at least twice as long. Of course, I haven't done both ways, but I know people who learned it the way you've said, and it took them much longer to put it together than it took me to work it up to tempo.
For a straight answer to the question in the original post . . . I would recommend waiting. Of course, I don't know whether you're an amazing learner . . . but there are lots of showy pieces that are "safer" if you're looking to impress, and you have to be very musical to pull off the inner section compared with the fast parts of this piece, and in my experience, that takes lots of time and experience with the piano. I'm not claiming to be very musical -- the inner section is probably my worst part of the piece. Some days it's gorgeous, and some days it sounds boring as hell.
I'm 16 right now, and I've been playing the piano since I was four. I'm not as good as I should be, largely because I had two bad teachers and had to take some time off at a few intervals, but I have an excellent teacher now. We decided in October last year that I was going to take my ATCL in June 2005, and he gave me the list of pieces to choose from. I actually selected fairly randomly, but I wanted to play the Fantasie-Impromptu because I'd heard it once before and it sounded fantastic. We ended up working on a Mozart first, and I only reminded my teacher about the Fantasie two weeks before I was headed out of the country for a month
He looked at me and said: "If you can't get the rhythm during the first half of this lesson, you had better forget about it."
I got the rhythm.

I had to relearn some of it when I got back, but all in all, it probably took me about four weeks of hard practice (as in every spare moment) to get this piece up to the right tempo. And then I taught myself the middle section . . .
Truth be told, I'm pretty pleased with the amout of time I spent on this, because I've seen friends spend three or four months working on it and not be able to play at the speed I can pull off (on good days, as I said

)
The ONLY reason I was able to do this was, as I said, because I NEVER took the hands separately until I didn't have to think about how the rhythm was fitting together. I had to tape it several times to make sure it was correct . . . but now, I don't even have to think about it. It just goes.
If you get both hands up to speed separately, you will ALWAYS have to think about how the rhythm fits together. It's also going to be a problem at the fifth measure, where the right hand comes in -- you're going to want to play the right hand in such a way that the sixteenths are the same value as the left-hand notes had been up to this point.
I guess my point is a) I would not recommend starting this piece right away (Fur Elise is currently something I am able to sightread at a moderate tempo) and b) if you intend to do it anyway, your proposed method will make it much harder. If you're set on learning it, at least go about it the most productive way.
And if you find yourself frustrated . . . don't say we didn't warn you!

Ooh . . . and I just saw this:
Breadboy: 8. I'm not going to piddle with a Mozart sonata... I'd rather get to the edge and be playing Godowsky's passacaglia in a few years rather then be worried about the easier stuff now. I always have time to go back and pick up the easy stuff.
This is definitely a sign to me that you're not ready for the Fantasie-Impromptu. If you think Mozart is easy, you need to have a serious discussion with your piano teacher . . . or since maybe it's their fault, with musicians at large.
Admittedly, you can learn Mozart faster than you can learn, say, Ravel or Bartok. Possibly also Chopin, but it varies. But performing Mozart is one of the hardest things you can do. Many professionals don't DARE to perform Mozart because you have to be ABSOLUTELY accurate. It doesn't sound like you have played any Mozart in the past . . . or even heard much more than maybe the K.545, or you would realize this. I suggest listening to K.475 and K.457. These are considered to be at the same level as the Fantasie-Impromptu, and there's definitely a reason for this.
Even after three months of learning, you have to have SOME understanding of musicality. If you can't see why Mozart is demanding . . . if you really think it's something to "piddle" with . . . that's, one, a sign that you will probably never play Mozart well, even if you "go back" to it later, because you don't understand it . . . and to me, is also a sign that . . . maybe you aren't really cut out for this.
I'm not trying to be offensive (at least not MUCH) . . . but seriously, you don't patronize Mozart. That's just stupid.