Check your motherboard specs first for the capacity per slot. Newegg and frys stores have competitive prices for ram. Avoid high latency stuff, which won't post with many boards. Pricewatch is also a good source for ram.
Thanks for your answers.
Finally, I have found and bought a 1 GB Kingston memory DDR PC2100 equivalent to PC-256 for 32 $.
Next year, I will buy a new laptop.
Thanks to all participants