I usually download several things at once just to make sure. Normally I'd do a developer's kit from Apple, because, like Microsoft, they use Akamai caching which gives you huge potential download speeds, and I grab a FreeBSD install ISO from the server that gives the best speed according to /usr/ports/sysutils/fastest_cvsup and maybe something from somewhere completely unrelated, like ... um google video?
All throughout this, I'd use net_snmp and MRTG to monitor my bandwidth on my server/router. As long as I can sustain over 12Mbps, I'm happy. My connection is supposedly 24Mbps, but I rarely peak close to 16-18Mbps.