Thought I'd add an example to this post too, just so you'll have an idea of how difficult it is to get a decent shot of sports at night. This image also required quite a lot of post processing to make it usable.
This was taken just after sunset and this was a practice field so it didn't have a lot of overhead lights like you may have on your soccer field.
This was taken with an ISO setting of 1000 and while I have noise, Canon does seem to handle noise better than most other camera manufacturers. This was also taken with a very good lens that has a big opening.
Canon 30D - Canon 70-200mm f/2.8 - 1/320s - 70mm