Most often, people will keep their receivers and upgrade the speakers. A really good set of speakers can make a mediocre receiver sound great. But a bad set of speakers can make a great receiver sound bad. If you find a receiver that will meet your current and future needs, the nspend any extra money on the speakers. For the front sound stage (R/L/C) it's always best to buy from the same mfr because of timbre. That's not a hard fast rule but if you buy speakers as a set or separates (R/L and then C) chances are they are made out of the same materials and design so they will resonate very similar so the sound is much more balanced. The sub-woofer can be made by anyone. The sides/rears don't have to be the same as the fronts because 80%-90% (my estimates) of the sound comes from the fronts and the rest are just effects and atmosphere as I've mentioned. You could buy a 5.1 set of speakers and only use the fronts. Then, if you decide to upgrade the fronts down the road, you can always use the extra side speakers if you go 5.1 or even use the front R/L and rear speakers if you go 7.1. You didn't say what kind of speakers you want. Floor standing or bookshelf. There are many more options for floor standing but the prices can go up very rapidly.
I'd decide what you want in a receiver and then once you pick a receiver, that will give you an idea of how much you can spend for speakers. Or, look at the HTiB's from the three mfrs I suggested and pick up one that you can reasonably afford. The receiver will be nice and the speakers should be good enough to get you by until you can upgrade. I would stay away from HTiB's by Sony, Panasonic, Samsung, etc because quite often the speakers have proprietary connections which makes it difficult to upgrade because of the way they connect to the receiver. They can make nice systems just be wary of the receiver speaker connectors. You want to look for 5-way binding posts because those can take any type of speaker connection like bare wire, banana plug, etc.