L have my pc Dell optiplex Gx520 it has a problem of display and it having six beeps

you should replace the motherboard or the power supply of your computer. to determine which one to replace is a simple test. disconnect all that is connected to the motherboard and if the problem is still there, then the problem is the motherboard.

It in fact may be an alarm, over heating?
If it beeps in groups, like TVs do in an error mode, look up what it means on dell's web site.

the internal speaker is only for beeps. These beeps are constant or short and only happen when the computer is trying to tell you how its about to die lol.

Buy some speakers. I would highly recommend a 2.1 system from logitech.

http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010370060%2050001080%201090608149&bop=And&ShowDeactivatedMark=False&Order=PRICE

The exact meaning of the beeps for Dell depends on their... rhythm.

http://www.nullamatix.com/dell-diagnostic-beep-code-troublshooting-chart/

Six beeps might indicate a simple CMOS failure caused by a bad battery, which is easy to replace.

checked replacing ram and the motherboard that should fix ur problem or else try booting ur system without modem card plugged it and also by pass surge protector .

Try disabling your onboard graphics in the BIOS. Some BIOS allow this. If you have a sound card, disable the onboard sound too. Count your beeps. they contain a code. Go online and "Google" bios codes. This should give you a starting point to diagnose the problem.