Sadly that 512k is a bit of a long shot, even 256k is optimistic rather than probable
Here's the website I used for those who want to check:
http://www.samknows.com/broadband/checker2.php
My stats come from being at most about 1.4KM of cable away from exchange (add about 25% to your straight line distance for approximate cable route length - will vary).
If you're within 2KM of cable, 8Mbit is all yours most likely
Out to 3KM you should be good for about 5-6Mbit, 4KM is good for 3-4Mbit, these are guesstimates and things vary dramatically, so don't blame me if in a few months your 3.5KM line that I suggested could be good for 4Mbit will only do for 2
