dell dimension 4600 - constant, rapid beeping on startup

i have a dell dimension 4600. i was in windows xp pro, everything was fine, then it began behaving as if the enter key were being held down. i rebooted(had to hold down the power button and do a "hard shutdown"), and very shortly after restart, i heard a constant, rapid beeping sound. checked the back of the tower, A & D lights were yellow.

below is the diagnosis for yellow A & D lights from support.dell.com:

tried unplugging power from disk drives 1 by 1, only change occurred when all drives were unplugged except 3.5 floppy drive, whereupon there were two short beeps - only the D light was on - as expected, "pri/sec master/slave not found" error messages on screen: "F1 to continue, F2 to enter startup" - tried F1, nothing; tried F2, entered startup, but continued behaving as if enter key were being held down, but repetitions occurred at a slower rate - tried different combinations of hard drives plugged in 1 by 1, no go - tried resetting CMOS jumper on motherboard, no go - tried removing battery for 5 minutes, no go - 3 out of 4 RAM slots filled, tried pulling them out 1 by 1, only change occurred when all 3 were out: slower, lower beeps (1-3-2) - obviously corresponded to error code on support.dell.com: 1-3-1 through 2-4-4 - DIMMs not being properly identified or used. apparently, that problem superseded the original problem.

i don't have any other ideas at the moment - i might swap the power supply(or maybe processor, 2.4ghz) if i had a spare one. i also have a working dimension 8200 w/ 2.0 ghz processor, so i might try swapping the power supplies(or processors), but i'm hesitant to do anything drastic. i'll wait for a response here first.

thanks for your attention.