This weekend (just gone) was a good meeting to attend as there were some quick cars (running 6 secs/240mph) right down to street cars, but it is generally not that busy, spectator wise. Look out for it next year, its called the Summer Nationals.
As for the rest of the year at the Pod is concerned, the top event in Europe happens there in September 11th - 13th. It will be rammed! If you go on the saturday it will be a shade quieter and the top cars have two qualifying sessions. If you go on the sunday, make sure your there before 8:30, or expect 1 helluva queue to get in. If it's raining, stay at home
Another excellent event at the Pod is the Mopar Nationals, July 25th/26th. This is a well attended event with some quick cars too.
Finally the national finals happen Sept 26th 27th - again, a well attended event competitor wise, maybe not so spectator wise which is great for the camera!
I agree with the advice earlier - stand high on the spectator banking and shoot the far lane, and 300mm is more than enough as your so close to the track. Don't be frightened to wander round niether - obviously stay the right side of fences etc, but there are so kool locations to shoot cars from, like the top end of the track, mid track. the pits and both sides of the track.