Dear Mehdi ,
As Michael and Ian saied you can't generate more than 8 characters on this type of LCD due to the limited CG-RAM which need 8x8 bits of ram for each custom character and 64 bytes total for 8 definable cahrs .

Sometime ago i was working on this and have written a VB program to design Persian Font characters for these Hitachi HD44780 character LCDs (5x8 pixel per char), and hardly ended up in 78 diffrent chars .
then i wrote an assembly code (595 bytes of hex code + 628 byte char code ) which divided the typical 2x16 char LCD to 4 segments of 8 character .
code has been generating the chars for first secion then clears this section and go for the next one , and so on .
This refresh maded the cpu quite busy and the contrast level was terrible (too much clear screens) but it worked !, then i tried and wrote a better code which dynamicaly procesed the text that wanted to be displayed , based on the text it divided the screen to less than 4 sections .

after that i gave up using char LCDs for full screen displaying of more than 8-16 custom characters .
you can use Nokia 3310 LCDs (84*48) which is easily available through the market , their interface is I2C and easily bitbangable , available for only 3000 Tomans (~$4).
here is a link to a sample project where 3310 LCD has been used (Yammp Mp3 player) :
http://www.myplace.nu/mp3/nokialcd.htm