Unable to enable unlock 512MB RAM of Model B

Question

I want to set RAM to 384MB. I've tried several things to try and get the RAM above the 184MB shown below by running free -h

             total       used       free     shared    buffers     cached
Mem:          184M        73M       111M         0B        10M        38M
-/+ buffers/cache:        23M       160M
Swap:          99M         0B        99M

I followed the instructions here, there was no change. I also added the following line to /boot/config.txt:

gpu_mem_512=128

Again, no change. I also tried following this guide, but it returned error 404. (I can't see the file arm384_start.elf in my /boot directory).

What do I need to do to get it to work? I'm actually wondering if I loaded the correct image onto the SD card to start with - is it possible that I loaded the Model A's image? How can I tell?

Thanks

Edit: As per the link posted by @avra, the contents of /proc/cpuinfo is as follows:

Processor       : ARMv6-compatible processor rev 7 (v6l)
BogoMIPS        : 697.95
Features        : swp half thumb fastmult vfp edsp java tls
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x0
CPU part        : 0xb76
CPU revision    : 7

Hardware        : BCM2708
Revision        : 0003
Serial          : 00000000da25038e

So since Revision < 5 it would seem it's the 256MB version?? It's odd since it has 2 USB ports and an ethernet port! Seems to conflict with the RPi FAQ:

What’s the difference between Model A and Model B? Model A has 256MB RAM, one USB port and no Ethernet (network connection). Model B has 512MB RAM, 2 USB port and an Ethernet port.

Answer 1

Model B Pi's had 256M of RAM initially, and this changed on 15th October 2012.

Your Pi was manufactured before this date and has 256M of RAM as indicated by the board revision.