I believe there were only 3 versions of the X51 (R1, R2, R3). If you enter your service tag into the Dell Support website then it will tell you for sure.
The beep codes are just a guide and don't always give the correct answer. A lot of the time it can be a motherboard issue which is located at the point of the other piece of hardware. For instance it could be the memory sockets on the mobo which have failed so the machine says it's a memory fault. It could also be the CMOS battery. It should give the CMOS beep code but the beep codes are a little funky in the X51, from past experience.
You could try to remove the CMOS battery and bridge the connectors on the mobo with a screwdriver. Make sure the machine is unplugged of course. This will reset the BIOS. Don't do anything that you're not comfortable with though.
My guess is that your mobo has died but hopefully it's something simple.