Optiplex 990 MT - Upgrade i5 CPU To i7 CPU?

It is a mini tower.

Does anyone know how I find out the socket type that currently holds the i5-2500 processor in the machine. Once I know the socket type then I can hopefully find out if it will accept an i7 processor.

I am going to upgrade the PSU anyway so that is not an issue re bigger PSU required and I can easily upgrade the BIOS if there is a newer version available.

Some versions will require a bigger power supply and some versions will not work at all EVER due to a different type of Socket.  There is no One Size fits all answer because there are a limited number of validated processors and there are many TYPES of 990 aka Tower Desktop SFF USFF.  You should always update the BIOS to the latest version BEFORE trying a newer processor.

The INTEL Q67 Express Chipset  Cougar Point has LGA 1155 socket.

LGA 1155, also called Socket H2, is an Intel microprocessor compatible socket which supports Intel Sandy Bridge  microprocessors.

Sandy Bridge based processors officially support up to DDR3-1333 memory.

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The Optiplex 990 is now currently being sold with an i7-2600 so it is likely to work just fine in your system.  That would also suggest that the stock power supply would be just fine with it as well.  It would be advisable to first upgrade your BIOS if it is not the most current version available.  And you are a gentleman for not correcting someone that missed the MT/Minitower indication that you included in the title of your original post.

That would be a risky assumption.

Dell can have several models and wattages for power supplys based on what processor is installed.

Faster cpus use more power thats basic.

Intel® Core™ i7-3930K (12M Cache, 3.20 GHz) FCLGA2011  is never an option.