Two Intel E5-2643 (4 cores each) or single E5-2690 (8 cores) for Oracle 11.2.0.3 on Oracle Linux
Performance will be very similar. GHz difference is not essential because e.g. E5-2690 has 3.8GHz Max Turbo Frequency vs 3.5GHz on E5-2643. You can find full comparision here:

http://ark.intel.com/compare/64596,64587  

The big thing may be licensing. If you are using Standard Edition or Standard Edition One per CPU licensing they are licensed per socket. So in case you buy second E5-2643 you will also have to buy one more Oracle license. Of course that it not the case with the Enterprise Edition.

If licensing is also not an issue then you still have two decide between two choices:


buying 2nd E5-2643 will be just cheaper now. Why not to save some money if everything else is the same?
if you buy E5-2690 and after some time you will hit CPU bottleneck again you will have an option to buy 2nd E5-2690 and once again double the performance.


Also with such amount of memory you really really should use Huge Pages. Of course that means no more AMM and back to ASMM. But especially if you have many connections to Oracle the difference can be really big. You can read about configuring Huge Pages e.g. here:

http://www.oracle-base.com/articles/linux/configuring-huge-pages-for-oracle-on-linux-64.php