electric piano

Hi expert
In computer many time that by cheap computer at moment but it more per formance than expensive computer last year in sound like piano it can happen the same

Pianos change each year, but not nearly as much as computers. Computers have a very quick turnover and "life". Pianos don't change that much each year. I would feel comfortable buying used.

The first full size electric piano I bought was a Roland KR-350 - at the time I got it it was 8 years old and it still plays well (better than I do!!) - buying old isn't a problem - there is no "upgrading" like with PC's but you do need to make sure it works :)
Also go and try some in a music store so you know what extra money will buy.  One of the things worth considering if you can fit into your budget is key weighting so tthe keys on the keyboard feel like piano keys when you play them rather than lightweight bits of plastic!
 
If you just want to play the keyboard through your computer there are plenty of bargains to be had at the moment as USB midi boards are being replaced for new boards that have Windows 7 support.