If what you have now is "very slow" then I would be "very fast" to replace it, with a Mac or a PC, but replace it fast. Everyone here prefers a Mac, I sure do. Your quality of life will actually get even better that it already is now.
As far as new models are concerned: Its impossible to predict when one will come out, don't worry about it. One consideration: adding memory after the fact costs a lot more than buying it up front with the new machine. You might consider a memory upgrade AND a service policy up front when you get something.
As far as discounts, price reductions, special promotions etc. are concerned: Yes, Mac usually does offer something worth $100 in rebates (not price reductions) that have to be applied to additional purchases such as a printer or a iPod etc. Its not a pure price break they do.
If you deal with one of their own stores, I think they allow small discounts for students or faculty with an ID. Figure about $50. I think. Those discounts are available all year, any time, all the time. So, once again, not much reason to wait. The online Mac store has a separates so-called: "Educational Store" and you can see what the price breaks are from there.
Finally, there is transferring the data and files from your PC based laptop to a new mac. The Mac company stores do that for free. An everyday retail store wants a much s $80 to do that.
Finally, finally, do you have a backup strategy? Naturally, you won't need one. (That is until you are about to turn in a very large project and there are only a few hours left to go and you have been pulling some all-nighters).
Good luck, hope it goes well for you.