It's best to use four tires of the same make and model. However, nobody likes to get rid of two perfectly good tires when the other two need replacing. So if you don't want to do that, you can replace only the two tires that need replacing. If you're going to do this, your best bet is to make sure of two things:
1. If you can't, or don't want to, replace them with the same make and model, then replace them with a make and model that offers similar performance characteristics. (For example, don't mix two supersticky extreme performance tires with two not-so-sticky tires like budget UHP tires or all-seasons.)
2. Choose the make and model of tire that best meets your needs, so that when the other two tires need replacing, you can do so with the same make and model. At that time, you will once again have four matching tires of the same make/model.
So if I were you, I would replace only the rears, with a set for which you will eventually buy fronts as well, when your fronts need replacing.
I would NOT buy the Bridgestone RE760 Sport, for two reasons. First, it's not available in the 215/40-17 front size you will need when it's time to replace the fronts. Second, it's an "ultra high performance" summer tire, which is not quite as good a performer as your current stock RE040 tires. You can do much better, while still meeting your objectives for tread life, wet traction, etc.
I recommend getting two "maximum performance" summer tires. There are two such tires that come in both of the sizes that you need (215/40-17 front, 255/40-17 rear): the Kumho Ecsta LE Sport and the Pirelli P Zero Nero GT. Tire Rack prices are $100F/$112R for the Kumho, and $98F/$182R for the Pirelli, plus shipping. These tires will last as long as the RE760 and perform better, and the Kumho is actually less money, too. So I would buy two rear tires now, either one of these, and when the fronts eventually need replacement, get front tires of the same make/model.
