I don't know much about RIS except that it's also a pxe server. Generally ghost should work with any pxe server, but it is definitely supported under 3com pxe server. Advantages of using it with 3com is that ghost boot wizard can create boot images that are readily usable with 3com pxe server, and there are also articles on the forum on how to set up winpe to boot with 3com pxe server or dcl pxe.
When pxe booting, a machine will use dhcp to discover any available pxe server.
With 3com pxe, the mac address of the machine is added to a list on the server (I think the bootptab file). This can be done with one of the tools that come with a 3com pxe installation. This is how the server knows which machine to grab. If the machine is set only on that one server then it will be that server that delivers the image.
the dcl pxe server will just deliver images to any machine that happens to find it first, so if you have 2 of them on the same network, the image will come from the first one the client machine finds. Though i'm not sure how the pxe client discovers the server so I don't know which one will be used.