Sounds like keeping what you have is the way to go, based on matching them to the center speaker. $30 isn't much when you consider the effort and time involved in swapping them for the R50s.
FYI, Fry's had those R50s for $49 (I'm pretty sure) about a month ago. They might go back on sale for that price later on.
If that is the case, then they would be about $80 cheaper. The polks cost $180 a pair. If I replaced all four then I would save $160 at that price. They may even be that price B&M still, and if not they probably will be again as the holiday season is coming up. Right now they are $69 each at frysoutpost.com, and fry's electronics just recently started matching their own website finally.
Are the R50's in any way "better" in terms of raw sound quality? (At least, in the terms described in my signature, with emphasis on the highs which the monitor 30's seem to do well.) If so I'll just get them instead as I think I may want floor standing speakers anyways. I'll just have to find a different center speaker for them. And I'll use the $60 I save towards getting a better sub.
BTW, assuming this would be the way to go, which center would probably go best with the R50's? Preferably one that can be found at fry's electronics since I am going there. If not fry's then circuit city as they are on the way. Choices are mainly CSR, CSM, CSi3, CS1, and CS2.
The CSi3 in particular sounds like it might do better highs than the CS1, and it is the same price. If it would match the R50's, and so long as they won't in any way sound "worse" than the monitor 30's, then it would be worth swapping this setup for that one if I could save $60, and use that $60 for a sub that I already know to be better than the one I have.