Just my thoughts..others may differ. I understand your budget constraints. We all have them to varying degrees and that shouldn't be a reason for you not to enjoy your system. Being that you have speakers that you know you will eventually upgrade, I'd consider looking into a center speaker that you'd like to keep around long term. It won't match up right now, but think about something you may want to get down the line. For instance, you mention the Infinity and the Polk center, but are your plans to build around that with Infinity fronts or Polk fronts? If so, fine. But if not, it may be a waste. Just quickly, off the top of my head, I'd maybe look at eBay. Considering your on a budget, but don't want to sacrifice sound, I'd look at someone like paradigm, specifically their performance series. I found this cc 170 on there now for about $100
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=3276&item=5762554091&rd=1
What this does is it then allows you to add some Paradigms around it in the future, without negating the investment you've made up front for the center channel.
There are many others too. Like Axiom. You can get their center (brand new by the way) for $240:
http://www.axiomaudio.com/vp100.html
That would then allow you to build around it with some Axiom speakers.
There are plenty others out there. I guess the long and short of it is, if you think about long term, you could get a center that's a patch now, but then grow into in the future as your budget allows. That would be my approach.
Hope this helps some.