The “non-gui” clients I’m referring to are “command line” utilities, and their use is similar to working in the *nix shell (in WIN98 and XP you “run” ftp.exe as a “Dos” application), and I don’t really think that is what you want, since you said you really didn’t want to operate in the “shell”. I only suggested it as a “last resort” method since you mentioned 3 different computers/os - all have a “command line client”, so you would never be “stuck”.
The problems is that a “step by step” list of what to do is going to vary slighlty between these clients (though they are very similar).
Tthere are many free, and commercial, FTP clients out there that run, and operate the “same” on both WIN98 and XP. You can check snapfiles.com, or any other large file repository, for many choices, or you can check out the clients suggested in this Dreamhost WIki Article on ftp and clients.
The client I use is not recommended in your case, as it does not provide for “onsite editing”, which you indicated you wanted (though you can always do that through the Dreamhost provided webftp!).
I really think, though, that once you have the ftp program(s) selected that you want to use, you should become at least familiar enough with the shell to be able to uncompress “large” files. While it might seem like it is a lot to learn, it is not that difficult, and being able to uncompress large files quickly on the server is a huge help when you are trying to “move” a large application or database.