Kilobits per second is usually shortened to Kbps (or kbps). The lowercase b usually stands for bits while the uppercase B stands for bytes.
1 kb/s = 1000 b/s
1 KB/s = 1024 B/s
1 KB/s = 8.192 kb/s
1 kb/s = .1221 KB/s
To find your theoretical download speed for your line, divide your advertised speed by 10, this includes a small overhead:
53.333kbps / 10 = 5.33 KB/s
512 kbps /10 = 51.2 KB/s
So based on the above a 80KB web page will take 80/5.33 seconds (15) and on ADSL 80/51.2 which is 1.56 seconds. This does not take into account poor connectivity, someone else nicking your bandwidth, etc.