Keep in mind that the speeds you're reading when the download first starts likely aren't very accurate. Many applications don't have the best algorithms for determining download speed, and as a result the first few seconds of data suck. You may want to get a third party program which will monitor your network status separately to see what your real download speeds are.
Are there other people in your house sharing your connection? Are you sure they aren't uploading like crazy or something? The high ping time makes me suspicious.
If you find me on IRC at some point, I can help you try to figure out what is causing this.
There is also a chance that someone near you got a new baby monitor, set of cordless phones, microwave or other device that is interfering. you may want to go into your wireless router's configuration and set it to use channel 1 or channel 11. Most home wireless devices default to channel 6, and if you live in a densely populated area you will be competing with them to get your signal through. You only have to change it on your access point/router, your client devices will all automatically switch when they see the access point change.
