Matthew Moynihan clarify your 6 beeps. Are all the same length or do you have something like 1 beep followed by 1 more and then a burst of 4 beeps? Also check your system lights. Your power button light and hard-drive light may indicate a computer problem. Your Dell Optiplex 745 should also have some diagnostics light to help you troubleshoot a problem. Your computer should have four lights labeled "1," "2," "3," and "4" on the front or back panel. The lights can be off or green. When the computer starts normally, the patterns or codes on the lights change as the boot process completes. If the POST portion of system boot completes successfully, all four lights display solid green. If the computer malfunctions during the POST process, the pattern displayed on the LEDs may help identify where in the process the computer halted.
NOTE: The orientation of the diagnostic lights may vary dependent upon the computer type. The diagnostic lights can appear either vertical or horizontal. Let us know exactly what you see or hear.
6 beeps is commonly a POST beep error for ROM BIOS error. Remove your RAM and try one ram chip, then the other, individually. Also try different slots.