That's not right either...
20 gigs is about 20,000 megabytes, which is about 20,000,000 kilobytes. On a 56k modem (that's a maximum download speed of 56 kilobytes per second), assuming constant maximum speed on the connection, that works out to about 357,000 seconds, about 6,000 minutes, or about 99 hours; just over 4 days. Completely doable in the time frame the OP specified.