Hardware Limitations?

I am needing to build an asterisk server that can handle 300-600 concurrent calls. I know that some OS’s have limitations on how much hardware they can use. Is CentOS this way? Can i run AsteriskNOW on any server or is their some top outs that the OS has. I am looking at servers and would like to know if Xeon processor are fine to use, and if a server running 2 processors will do fine. I am also going to try to put 4 4port PRI cards from digium in there. I have seen some forums that say that you can not put 2 in a PC. then i saw others that say you can.
Also how much RAM can be addressed is it like XP that if you go over 4Gb’s the OS can not address it anyways?
I see that if you are transcoding it realy boggs down the server I know G.711 to G.729 is transcoding but is it transcoding if i am going from G.729 to a PRI channel?

I need to build a server and i really do not want to come out the other side and find out that i needed something else.

PRI channels are G.711, so G.729 to PRI is transcoding.

With that number of calls, you will need to know a lot about the nature of the traffic. I believe voip-info.org has some sizing examples, but they need to be interpreted in the context of how the traffic is handled.