As others have said, get the 5.1 (or 7.1) receiver first. AV receivers with true discrete 5.1 decoding capability also have virtual surround functions that allow you to enjoy a simulated surround effect from just two speakers. It's a huge step up from the Pro Logic playback you get right now.

Incrementally add your speakers, starting with the surround speakers. It's easy for the L/R front speakers to simulate a center channel effect (after all, the phantom center effect is what two-channel stereo recordings create to begin with), but much harder for those same front speakers to simulate sound coming from behind and to the side of you.

Center channels do matter in the sense that they are a discrete channel and it's best to use a discrete speaker for a discrete channel. But, when you get around to adding a center speaker, be sure that it properly voice matches your L/R front speakers. IMO, having no center speaker is preferable to having a mismatching center speaker.