Replace center speaker or L&R front speakers?

First of all, what is your budget?

In general, you should upgrade all 3 (left, right, and center) in order to match the timbre of the front speakers. Otherwise, if dialogue or action moves from left to right, the same person or object will sound different as it moves along the different speakers.

With that said, you also have to make sure that you can upgrade the speakers at all. Some HTIB have a proprietary connection that prevents you from upgrading to better speakers. If you were able to use generic speaker wire, then you're set.

Hope this helped!
Josh

also, many inexpensive HTiB sets have amplifiers which expect to output to 2 or 3 Ohm speakers (thats how they get their outrageous power claims). If you attach 8 Ohm speakers, the apparent "loudness" may decline.

+1 to the Pioneers.

If you plan to make a next step, you'll want to upgrade the sub. The solid Dayton 8" sub is the minimum you'll want to spend for it to be a worthwhile upgrade.

+2 to the Pioneers. I am running 3 of the c21 LCR setup and they sound better than most $600+ setups Ive heard.

I would buy a used set off the classified area here, ebay, or audiogon. People are always upgrading and there are deals to be had on used speakers. Just my input.

Yea, going higher won't short your receiver, but it will force it a lot more. It'll probably decrease the life of the receiver, but you'll need (want?) to upgrade that once you have the funds.

Pioneers are a very good start for the money. Eventually you'll want to get away from your HTIB receiver so these would go well with whatever you buy next too.