Your hardware setup may not allow all that bandwith to actually go to your computer. In addition, Limewire is not responsible for your personal speed while on Limewire. If it's going slow, it's because you're downloading from someone who cannot upload at the speed you'd like to download, or your bandwith is being used by other activities, quite possible.

*chuckles* I don't think your cable modem is "54mbps".

Could you please post your specs as well.