As a DBA, which would you pick for your workstation? QuadCore 2.4GHz with 12GB ram or Dual QuadCore 2.0GHz with 6GB ram?
Get the memory. 'As a DBA' you're going to be doing I/O bound work more than CPU bound work. Assuming you have a 64-bit O/S the memory is probably more useful than the CPU cores.
If you are feeling flush, get some internal disks (SSDs for the particularly well heeled) so you can test stuff locally on the machine. I use XW9300s (pimped somewhat by adding internal SCSI disk arrays) for data warehouse development work with Windows XP and SQL Server developer edition. This works surprisingly well. A 64-bit build with 12GB of RAM would be even better, and a similar trick could be done with Linux and Oracle.
I would go for the Dual Quad core and I will tell you why. Put simply the dual quadcore system will continue to be useful as it has enough processing capability that it will not be outpaced by the tendency of software to become more bloated over time.
You can always buy more memory when it is not running fast enough for you. And to be honest, the memory sticks are getting to be pretty cheap now.
Your system will have a much longer service life if you go with the one with the greater processing capability.