I only did 1 soccer game, with my niece's team, and let me tell you, it's not for the faint of heart - day or night. I did my one game with the 70-200 f/2.8 IS, and I found it to be short.
The game is a very fast moving game with lots of running and kicking, and for that reason alone, you will need a 2.8 or faster lens.
I suggest that you save more money in the pot to be able to afford the sigma 120-300 f/ 2.8 IS ($2200 at sigma4 less brand new, $2700 at B & H - some say you can buy used around $1800-$1900 - I've been looking for a couple of weeks now - I haven't come across one yet). 300 is still alittle on the short side for soccer, but realistically, you want to spend $6500.00 and up on a 400 f/2.8? Didn't think so. You can put an extender on the sigma (several folks here tell me that you can use the canon extender on the sigma), to get 420 at f4. That's pretty reasonable for day use. You can use the f/2.8 at night, but it will be very difficult for night stuff, even at f/2.8. I, myself, am saving for the sigma and hope to get one for next LL season.
For night games, once you are able to save more money towards a separate lens for night work (and use the f/2.8 in the meantime), what I am suggesting is that you get perhaps the 85 1.2 L (this lens also comes in a non-L 1.8 version), or the longer 135 f/2 (??). I know these are terribly, terribly short, but these are faster lenses which will serve better at night, but you will be sacrificing focal length and will have to wait for action to come to you. I've tried shooting at night with an f/2.8 with a 30D, and I have to push the iso really high to 1600 (even tried some with 3200) to get a decent shutter speed, and then you have to deal with noise. Night action shots are tremendously hard, unless you have a 1-d series camera. I pretty much have given up and won't shoot night ball games.
I know these suggestions are way over your budget, but I am trying to save you the headache of buying inadequate equipment, then realizing your mistake, then you're losing money on selling it off to be able to buy the right equipment.