Seriously, if you haven't already, set up a track and race with some friends. I just recently started doing this, and it's totally awesome!

Its a completely different experience than darting around your kitchen floor. You'll need all the parts you have to experiment with different setups for different track configurations. Now I'm finding that my driving skill is what I need to upgrade. It's cool if your friends keep getting better too, then you always have to keep working to stay on top!
And just a note: you dont need $300 worth of foam to have a track. We race around the legs of a giant confrence table at work, and I've even set up a course using soup cans to mark corners. Any big low-pile carpeted area is great. I'm looking at buying some of the $2 packs of zip-zap road cones and setting up a course autocross style