You are correct. Dell as well as all OEM manufacturers write their own version of the bios, which is limited and has no features. You can't do anything with an OEM bios.
So you have a couple options. It appears to me that the 570 will support CPUs up to the Phenom II and Athlon II. I suggest looking on the dell site under support for your computer in order to make sure whatever CPU you decide to get is compatible.
Another option is to purchase a motherboard to replace the limited dell one. Although more expensive than a new CPU, it will open up all bios options.
Also a note about unlocking cores: Unlocking cores means that you would be operating outside of what AMD intended. AMDs semprons are made from dual core athlon IIs that had a bad core. If you unlock one of those cores, it will be unstable and lead to system crashes. The reason you can sometimes unlock cores is this: If amd does not have enough bad athlon chips to disable the bad core in, they will disable a good core in a good chip to keep up with demand. There is no way to know which one you got.