I'm a guitarist, too, and that Yamaha or a $200 Casio would probably suit you well for now. In fact, with some careful EQ and DSP, these cheapie boards are capable of some very useable backing sounds. Just don't expect the acoustic sounds, like a piano, to be anything convincing. I made the mistake last year of using my Casio's piano sound for a key part in a recording I did--now every time someone hears that song, I'm tired of hearing that I should find a real piano. Now, I just use the Casio for symphonic strings, synth, and such. Very acceptable for a cheap fix although I'm eyeing something better, especially with stronger piano sounds, within the next year or two.
Best,
J.