I've got a slingbox, and as Sharkykzn says you need an upload of at least 1mbps to get a good picture and sound. Anything less then 500kbps and the picture becomes 'blocky'. Thats for a standard def image - for HD you will need probably a 7mbps+ upload. You can watch the stream on your laptop/PC and if you can hook it up to your tv and watch on there. Or you can also purchase a 'sling catcher' which you can permanently hook up to your tv direct. Has a remote etc
The only thing i'll contradict with Sharkyksn is that you don't need a static IP, or i never did anyway...
If your slingbox is in the UK and suitably close to an exchange use o2 or Be Braoadband as your isp, both offer 2.5mbps upload speeds
Oh and if you have an iPhone and purchase the slingbox app you can watch on there as well over a 3G network (and the quality is more than adequate)