Use Ethernet of course. Internet speed doesn’t matter much, 1 Mbps is more than enough for SF, anything more won’t make a single difference, it’s all about the latency (ping).
On your PC (also connected via Ethernet hopefully), go to www.speedtest.net, and check the connection on your local ISP server.
How much is the ping? If you’re getting high ping # even to your ISP, then you’ll be in a pretty bad situation for online gaming.
< 8ms is excellent
< 15ms is great
< 20ms is good
< 30ms is decent
< 40ms is OK, playable but you’ll start to really feel the online bullshit past this point
50 and above will start to hurt your SF experience pretty bad because the ping to anyone else will be even higher than this which is only the route to your own ISP. To give you an idea how this effects gameplay, every frame in SF = 16.6 ms , so 50ms latency results in gameplay 3 frames slower than normal offline gameplay, if that number is to your ISP alone, opponents will have even higher depending on the destination and internet route from your ISP to them.
USA is huge though so that’s why bad online gameplay is common there, usually the further the person you’re playing against, the more delay there will be in the response time and more lag, e.g East coast player vs West coast player. There are some exception that even close by opponents will lag hard because the route from your ISP to them is slow.
Edit: forgot to comment on the skipping/jittering part, if you’re on Windows, go to Start > All programs > Accessories > Command Prompt
Then type this command and press Enter:
ping www.google.com -t
The proper response with a good internet connection would be:
1- Getting consistent replies without fluctuations in times (ping time in ms), time between replies should be only a few milliseconds apart, if it’s jumping up and down by hundreds of ms that’s Very bad.
2- You should get consistent replies without losing packets and getting the “Request timed out” message, those are Very bad for online gaming and if you see many of those it means your internet connection isn’t stable.
You can run this command for a while and then stop it by pressing CTRL + C to see the final stats (average response time, loss packets…etc)
ps: For both of those tests, make sure that you don’t have anything on whether on your PC or anyone connected to the the same network through your router, stuff such as streaming youtube or Torrents will mess these results up bad and they’ll have an equal effect on messing up your online gaming experience.