What are the maximum and nominal input currents that the Arduino Uno board handles through each of its power supplies (USB, jack, Vin/GND)? It doesn't say on the official page. The only hint it gives is that it has a 500 mA fuse for the USB connection.
I've verified that hooking up a 9 V battery straight to the Uno works at least short-term, I'm worried about frying it though. Do I need a current limiting resistor in series from the battery into Vin? Also, will using a battery affect performance?
What will help battery performance is not wasting a lot of battery capacity making heat with the 5V regulator.
Look into a switching stepdown regulator if you are really stuck using a 9V battery (like the square ones which have relatively little capacity).
Some examples, with different current capacities:
Then think about running from LiPOs, NiMH, some kind of rechargeable battery.
I purhcased a 1000mAH LiPO from www.gravitech.us for $12 and use it to power an RF remote control (had to ask for it, doesn't appear on their webpage) and use a MAX1811 charge control chip to recharge from a 5v source.