ADSL, 1.5Mbit down, 256K up.
I've had to implement some QoS, with respect to FTP and HTTP. I use mod_bandwidth to restrict (8Kb/sec) large downloads thanks to rotarymiata.com (AKA, Bandwidth hog). I do a fair amount of bandwidth, almost constant usage with respect to smtp/pop3/pop3SSL/smtpSSL.
I think you can be comfortable hosting at least 4 "high_usage" sites without issues on a 256K pipe.
Also note that I am using mod_gzip, which effectively saves me at least 200+ MB/Month
Hope this helped,
Nathan