My site has around 400,000 posts and 1620+ members the database is 60mb with gzip (without it is 220mb using invision power board), that is just the size of the forum database, the CMS is around 50 mb. I used to back up the database through admin cp but now its practically impossible as more often than not firefox crashes, now I use phpmyadmin, attatchments are stored inside the databse. As far as speed is concerned it is pretty decent alot of time but goes down considerably when the number of online users at one time go above 200.

Hey Scribbller. May I ask which site you run? The forum you have in your signature only has 65,000 posts. 400,000 is quite the big accomplishment! According to the vBulletin admin panel, the data usage is 320 megs, with 60 megs of attachments, 3 megs of profile pictures, and another 3 megs of avatars.

mysqldump (which I use religiously, set up in a cron job four times a day because you can never backup often enough) currently creates a 450 meg file. Web-based backups stopped working for me a long time ago.

It backs up every 4 hours, and archives a day's worth of backups.

Although I do not use the same software, I have similar solutions.

Cron job tar/gzips all the web site, dumps SQL db into file which also gets tar/gziped once a day.
I periodically download both files (the web tar.gz and db tar.gz) to my local machine.

Once the site is big enough, I will increase the frequency of backups, and FTP the files to a remote location right after the cron job. Hopefully this will assist me in recovery after crashes whatever the reasons are. I will use the grandfather/father/son rotation.