I have been to several F1 grand prix over the last couple of years including Barcelona and though I may be able to add to the comments that have already been made.
If you have an allocated seat then your movement may be restricted although moving about the stand on practice days is easier. Getting close to the track will give you the problem of the catch fencing. No matter what others have said it gets in the way and unless you find a suitable hole of the mesh is wide enough to get the lens through think of something else. If you can get high enough to take pics over the fencing that will be better but I fear that your 300mm lens may not be quite long enough.
If you have a general admission ticket you may find moving around to get a good spot easier. Just remember practice sessions give the best opportunity and more freedom, race day will be crowded and you want to enjoy the race.
I took pics at Istanbul as couple of years ago using a 70-200 and it was not close enough. I have been at Abu Dhabi (November 2009) and Bahrain (March 2010) and used a 300mm f4 usually with a x1.4 convertor. My pictures, including examples from these three events are on Flickr at
http://www.flickr.com/photos/garrellmillhouse/
I took many pictures at local motor sport events and used a high shutter speed but it stops action so much that the cars look like they are stationery, unless there is something else to convey action such as smoke from a locked wheel. For cars approaching you a high shutter speed will be needed but for more dramatic action the cars need to being moving past you. Barcelona should be sunny enough to allow you to use a shutter speed between 1/250 and 1/400 with enough depth of field to capture action without the need for a monopod. I would always try to use one but in grandstands they can get in the way.
If you get the chance to get anywhere near to the pits, such as the pit exit, you may be able to get some close stuff. I was lucky in Bahrain but it involved holding the camera above me at arms length, pressing the shutter and hoping for the best. You gets lots of shots of empty tarmac!!! In fact you are going to get lots of rubbish – I think getting 1 in 10 is good going!
Get out and practice panning on anything fast moving, just don’t cause a crash!! Cars on the motorways will only be doing about 70mph the F1 cars will be doing up to 180mph at some points – trying to follow them is not easy.
Wish I was going, Barcelona is a great city – you must visit Gaudi’s Cathedral (Sagrada Familia) and go to La Pedrera where you will get some great views from the roof and see the crazy chimneys – good photo opportunities).
Good luck.