2000 chrysler concorde amplifier

If your system used an external amp, then no, the OEM head unit will not work without it, and yes, you either needed a new head unit (and the proper amplifier bypass harness, which generally costs around $20) OR you needed a new amplifier and perhaps an amplifier integration harness for the OEM head (which can cost up to $100 depending on the vehicle.)  I'd demand that Circuit City sell you the proper amplifier bypass harness and install it as they should have done in the first place.  If the factory speakers cannot handle the power from your new HU or if they simply sound like trash (which is what I suspect you will find) then you may need to replace those as well.

I would recommend NOT buying a new or junkyard stock amp.  You'll be better off with a new HU and eventually you will want to spend the $300 to replace your OEM speakers.  I also recommend that you visit a few non-Circuit City car audio shops in your area as you may find your $300 can go farther than you think.

If it is in fact an LXi, then yes, it does have the upgraded infinity sound sytem. And in fact that is what the "i" in "LXi" stands for..

And actually, DYohn, my father bought a Concorde LXi in 2001, and I have to say, the Infinity set up that came in it was quite impressive. I doubt very much that it would stand up to the uh, "Hamfist Mobile" but as compared to other factory upgrades. i.e. the "Blows" upgrade you can get with GM vehicles, or the Rockford through Mitsubishi, I would say is outstanding.. Just my $.02....