Going the Arduino route is most likely the easiest way to start off, as there's a huge community around it. While I'm not sure if the performance is sufficient enough to do the sensor fusion on-board, but if it is, then all you might need to do is emulate a gamepad/joystick as a lot of games have at least some built-in support for them giving you something to work with. If it can't do the sensor fusion, but you still would like the emulate a joystick, it might be possible to have the Arduino enumerate as two different USB devices - one which transmits raw sensor data and receives the fused sensor data, and another that enumerates as a game pad and re-transmits the fused data as a joystick. You would have to write an application on the PC side that does the sensor fusion, but wouldn't have to necessarily interface it with the game directly.
If you want to go with the wireless route take a look at this kit from TI (
http://www.ti.com/tool/cc2541dk-mini#0). It's not the cheapest thing, but you get some nice Bluetooth hardware and a programmer. The bottom right jumper on the key fob is a breakout for some of the chip's peripherals such as the I2C bus. Also, the fob has a built-in accelerometer which might come in handy.
Perhaps even better is this bluetooth kit:
http://www.ti.com/tool/cc2541dk-sensor "...includes TI’s TMP006 IR MEMS temperature sensor, humidity sensor, pressure sensor, accelerometer, gyroscope and magnetometer" for only $25! You'll need to have a bluetooth transceiver for your computer, and write software to interface with games, but that price is really hard to beat!