Pyrrho...are you saying to buy a center instead of the sub or in addition to it?
Maybe I can post a picture instead of posting a link to it? Let me see if that will work.
I mean that it is very important for the front three speakers to match. Otherwise, when sounds pan across the front, the tonal quality would shift when going to the different speakers. So, when you buy the front speakers, buy all three at once. Otherwise, if by the time you get around to buying a center, the company stops making speakers that match the right and left, you will have to either find a used one, or you will need to replace your front speakers in order to get them to all match.
If you are wanting the best sound you can get in the end, and are willing to spend additional money in the future, then right now, find the best three speakers you can for the front, that are within your budget. If you have extra money, then consider buying a subwoofer or whatever, but at your budget, I think it would be better to save up for that later, depending on how good of a subwoofer is good enough for you, and how much you will be willing to spend later on it.
I recommend that you go out and listen to as many speakers as you can stand to audition, and think about which ones sound best to you.