Most Database server products multi-thread very well. So in theory, if the server is busy enough, the E5-2640 with more cores is going to be able to handle a little bit more users at once. But really, the difference is going to be very small and I wouldn't worry about it.
Normally, CPU is not nearly as important as I/O for database. Make sure you have a very fast disk system and enough RAM to cache most or all of the database (which lightens the hit on the disk for reads).
Or in other words, it won't matter if you buy the fastest CPU on earth if you don't have fast storage.
yes the 2640 should handle your needs perfectly, you can always overclock it to a 3.5 turbo easy and if you have a great deal of memory/RAM to go along with it you should be able to run very well... the cores is the most important with database servers(if we are talking CPU's) because it needs to process more things and division of labor is a very important thing in everything so the more cores means less equations persecond for each core, lowering total load, and speed up the transfer rate... and besides if you overclock to 3.5(or higher) then you should only be a little lower than the 2643 per core.
also if you don't mind me asking what is this for?
The OS for the disk will not be very important, 2x15K in Raid 1 will be sufficient. That should be a nice build, but if you can get more RAM (it's cheap these days) then the more the merrier.
Make sure your storage guys prep for high IOPS and use RAID 10 arrays without a lot of contention. A lot of time the storage people (I've been on both ends) can think in terms of gigabytes and not load and type of load.