Although I have no experience with the computers you listed, the comp in my signature was used from the public release to the expansion of Burning Crusade. It has an ATI video card with 32 MB of VRAM (ATY RV280M9+, if that helps). I quit playing because I was getting frustrated with my screen locking up, the case getting hot, and my frame rates below 10 fps. All my settings were at the lowest. I am waiting for an update to the Macs this summer before I resume playing *please give us a decent video card*. I had fun, but trust me, in busy areas of the game (there are plenty - esp. towns) you would get frustrated too. You can check your frame rates with CTRL R if you decide to try the trial. Although I agree with colocolo about gameplay being the best part of the game, I turned my settings up from time to time and waited for my video card to catch up, and was stunned by the difference. Considering how much fun I had enjoying the artistic visuals of the game from the low end of the settings, I am having a hard time waiting for the next Mac upgrades to view it from the other end.
This game is fun, addictive, and easy to start playing from the get go. My recommendation would be to max your RAM in which ever comp you pick to download to (if you haven't already). Your best ability to gain knowledge will be from Blizzard's own home page site for WoW, and once you begin aquiring quests and items thottbot.com, wowwiki.com, and allakhazam.com will help. Otherwise it's best to learn as you go and use the chat menus to ask questions of other players and become a member of the server community.
Also, while I'm at it:
-Dial up internet sucks for downloading patches
hopefully you don't have that problem!
-I switched later on to a PVP server to get the full experience of the game (didn't try the RP servers tho) and did not regret it even with my poor comp performance.
-Play both Alliance and Horde, you won't regret it, though you may get flamed if you admit to it.
