Actually, you are probably at about the right speed.
Your internet is supposed to be 2.4Mb/second (Megabits per second), because if it was megabytes per second (MB/second) it would be faster than anything SBC would offer. However, your download speed is most likely measured in KB/s (Kilobytes per second).
There are 8 bits in one byte (at least for your purposes), so 2.4Mb/s / 8 (to convert to megabytes) = .3MB/s * 1024 (to convert to kilobytes) = 307.2KB/s--pretty close to your download speed. This would also explain why streaming video works so well, it is actually at 1250Kb/s (Kilobits per second), which is lower than your connection speed.
If this is the case, you are very lucky; usually speeds are a bit lower than advertised. Go to the link below to find out your real speed (it will give your results in bits).
If your download speed is 334Kb/s (Kilobits per second), than contact SBC and complain; 334Kb/s is much too low if it is supposed to be 2457.6Kb/s.
Actually, 334Kb/s IS good enough considering the highest speed available to me is less than one-tenth of that! : )