Which value and type of capacitor?
It depends on the input impedance of the amplifier. The input impedance forms the "R" in your high-pass filter. If you assume an input impedance of 10K Ohms (or higher) a 1uF capacitor will give you a cutoff frequency below 20Hz, which is generally considered the lowest audible frequency.
An RC low-pass filter is enough?
That's not so easy to answer... The higher the PWM frequency the more filtering you'll get. If the PWM frequency is much-much higher than the highest audio frequency you should be able to get away with a simple filter.
I'm not an expert on PWM audio and I'm not sure if you can set the Arduino's PWM frequency high enough for high-quality audio, even with a good filter. And since the default built-in PWM is only 8-bits, you can't get high quality from it anyway.
PWM is more difficult to filter than "normal" PCM audio. With 44.1kHz PCM "CD audio" you can actually get-away with no filtering. The clock noise isn't nearly as bad with PCM as with PWM... I connected the output of a cheap soundcard to an oscilloscope once and I was surprised to see a stair-stepped PCM waveform with no filtering. The sound seemed fine to me! I assume the amplifier and speakers were filtering the sound and of course you can't hear anything at that frequency anyway.