Well, keep in mind that ISPs tell you "best possible" speeds, and you may not actually get anything close to that in reality - and there's nothing that you can do about that except complain, which is usually like talking to a brick wall....
512kbps isn't bad, but then that will be your DOWNLOAD speed, and it's your UPLOAD speed that matters for RTS gaming, so ask them what your UPLOAD rate will be.
Once you get connected,
do this to both measure your actual rate(s), and to determine the setting that you should use for Halo in particular.
The equipment (esp. any/all router(s)) you get/use, wireless (enabled & active, even if you're NOT USING IT) and simultaneous use with cable TV all can cause problems, as well as the number of simultaneous users.
I won't make any recommendations on off-the-shelf computers, but there's a reason why all of mine are builds of my own....
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