The location that I have my Slingbox base gets a 4MPS download and 1MPS upload.
If you want to increase the Slingbox IP you need to get more than 1Mbps for upload speed at the Slingbox location. This link might help you too,
As mntioned, the critical speed factor applied to the upload pacxkage you subscribe to at the slingbox hosting location.
In order to get a more pleasurable viewing experiance, you'll need to upgrade to a higher rated package from your ISP. The faster the upload speed, the better the experience. In order to fluently stream upto 1080i HD from your slingbox, you must have an upload speed of at least 3mbps.