Why would you need new calipers to replace stocks? Unless you do some serious racing, just changing to new pads and better rotors will work. I'm currently using Hawk HPS pads with R1 slotted rotors and they're noticeable over stocks. But if you absolutely must replace the calipers you can go with used Bremo's off of SRT's. But you need more then just the calipers, the knuckles, the rotors. Or you can go with Wilwoods that can be found here http://www.tceperformanceproducts.com/cgi-bin/brakes/cginews.pl?record=1
Seriously, I agree with Supreme G, the R1 concept rotors are amazing, and better pads with them, thats all most need.
On my RT I upgraded to a 3 piston caliper kit up front, It really was worth the 850 I bought them for, and I added the stoptech brake lines. All in all the whole setup is an econo-ass way to do it. But for 90% of people, the rotors and pads are all you'll need and you will be very happy.