Arno,

US$500 can only buy you a pair of Yamaha MSP5 biamp powered monitor speakers (US$224 a piece in Mars Music or Guitar Center) if you want stereo - they use them for keyboard demo in store, sound pretty nice. At home, I prefer a cheaper pair of Yamaha MS202 II powered speakers, 20W per speaker, (lowest price I found is $150 a piece at Mars Music, $300 for stereo), each MS202 II accepts 3 inputs (2 1/4" and 1 RCA) and has output (1/4") for external amplification (or direct box for mic hookup to house mixing console), with high and low EQ. I use them on stage for monitoring of multiple keyboards. The MS202 II's bass is not as nice as the MSP5 but it is very versatile, light (about 7 lb. each) and loud for its size and price.

Here is a price list (just for your budgeting reference) of some keyboard amps in pair:

Crate KX-15 $240 (very tiny amp, I like MS202 II better)
Fender KXR-60 $538
Crate KX-50 $540
Roland KC-100 $540
Fender SFX-200 $570 (discontinued, 82 lb, stereo digital simulation with single amp)
Fender KXR-100 $678
Roland KC-300 $740
Crate KX-80 $820
Carvin AG100D $860 (recommended by Donny very much)
Roland KC-500 $940

I hope you find the above somewhat useful,

Paul Ip
from Texas