Your image server must have enough disk space to hold the images to be installed on your client systems.
All of the clients that will use the same image should have hardware that is as similar as possible. Most importantly they should use the same chipset on the network device(s) and the same number and kind of hard drive(s) (IDE, SCSI, Mylex Hardware RAID, etc.) The hard drives may be of different capacities. Devices may be larger with no problem and can be smaller within reason.
For PXE installations, you will need a compatible tftp server running on the boot server (usually the image/DHCP server). H. Peter Anvin created the tftp-hpa package, which provides the required functionality. RedHat 7.0 includes this server in their tftp-server package, but the tftp package included with earlier RedHat releases doesn't provide the required functionality. Debian provides this server in the tftpd-hpa package. The atftpd server is also reported to work.
PXE network-based installations may also require a pxe daemon to be running on your image server. This requirement depends on the firmware used on the client side, and the capabilities of your DHCP server.