I'm assuming you have them wired together with ethernet cable.
If you want to try roaming between access points, you use the same SSID. If you want to manually select which access point you're connecting to, you use different SSID's.
If the access points can hear each other, then they can interfere with each other. I would use different channels (1, 6, or 11) to avoid any interference.
Yes. They should be on the same subnet 192.168.0.xxx as the DG834G, but with different IP's. The IP's are used strictly for managment and configuration. Make sure the IP's you select are NOT within the DHCP server address pool of the DG834g.
You do NOT need the setup CD. There are no "drivers" on the CD. Just point your web browser to the IP address of the access point and configure it manually.
Seamless roaming is not commonly available at this time without proprietary protocols. It really depends on your client computers wireless driver and not anything in the access points. Most clients will tenaciously retain a connection long past the point where it should have given up and searched for a better connection. It's not unusual to retain a connection from a distant access point, while standing in front of a much stronger one. Note that this assumes the same SSID for all the access points involved. Roaming will not work between different SSID's.
It doesn't matter what channel you select. When the bit error rate or signal level deteriorates to the point where the client software finally decides that it's time to roam, the client will scan for the same SSID on all the channels. The AP's can be on the same channel, or different channels. It doesn't matter. However, they have to be on the same SSID.
Unfortunately, most client do a miserable job of seamless roaming. You'll probably find that you have to manually disconnect, search for a new connection, and manually select the new connection.
See: | ftp://download.intel.com/business/bss/infrastructure/wireless/deployment/hotspot.pdf There's a section in there somewhere on channel and SSID selection (with the usual illustration of hexagonal "cells"). Although it's titled "Wireless Hotspot Deployment Guide", it really covers much of what's involved in setting up a conglomeration of wireless access points and wireless networking.
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If you use different SSIDs in all three wireless hosts, Windows XP still allows you to establish a specific priority scheme for those wireless hosts on each client computer and automatically connect to the first available wireless host in the priority scheme. This can come in handy -- for example, I use it in a certain public library to favor the wireless host that generally works best for me.