Dave makes a very good point.
Last summer, I also felt the urge to add an amp for the first time. I bought an RPA-1. The first unit power surged my speakers and stopped working on the second day, but the replacement has worked fine since. I'll say this - there was a noticeable improvement in my system from having the extra power at louder volumes. The sound was much less strained and more clear than it had been when powered by my Pioneer receiver. The RPA-1 is a nice unit.
And then I started to wonder...would it sound even better if I was also powering the center channel separately? I considered many options, agonized for months, and then finally went with an IPS-1. Now I have more channels than I need, and two amps. Before I start to sound like an Emotiva fanboy - the IPS-1 looks nices, there was a marginal improvement in my sound, but I'm not very happy with the amp itself (it hums loudly) or Emotiva anymore based on their lack of service.
My points? First, once you amp the front speakers, you may very well get the itch to amp the others - especially the center channel. Second, if you decide to amp the other channels after buying an RPA-2, you have to add another amp and/or try to sell the RPA-2. Third, I don't recommend Emotiva based on my experience with them. So, if you can get a 200W 5-channel amp for $500 right now, and if it's a nice one (I'll have to let others chime in on that because I don't know anything about that brand), then I say go for it. 200W is a good power number, and 5 channels gives you enough channels. If your friend will let you try the amp out before buying it, then even better! IMO, you should be able to tell right away if your system sounds better with the amp.
Just my two cents, and good luck!