Hardware parameters on modems, 343Mb = 34.3 MB ?? Which modem to buy?

I have Time Warner cable, confirmed the package is 50MB download speed.  I need to find a cable modem instead of lease theirs.  

I call Time warner and ask for tech support, got someone in sales and asked which unit should I buy that supports my 50MB rate, she pointed me to their reference page, ok.  she says you should get one that supports 100MB download speed in case you want to step up the rate.  ok, so I pick the first one and the spec says it handles up to max 343Mbits/sec Modem speed.  I say but Im told by Staples sales that you divide by 10, so 343 Mb = 34.3 MBytes.  she didnt know what I was saying, dont think about it that much she says, if its on the list? its fine, listen to what I'm saying.  She said  1000 Megabits to get 100 Megabytes or something like that;  

sure its 8 bits to a byte, but then overhead for ECC or ?? I suppose.    I've worked with communications equipment and old modems 

but bottom line, what cable modem is needed to be able to communicate with TWC at 50 MB ??  What is the way to figure out how to get a modem to communicate at that bitrate?

thanks