The best advice is to use it as is until you can buy a new computer. That model is 9 yrs old now and it would not be smart to upgrade it now. Any upgrade would still be old tech that would not make a difference. 

Depends on the Size of the unit.  USFF Does not support Quad Core CPUS.

https://www.dell.com/downloads/ap/products/optix/optix_760_tech_guide.pdf

Processor Upgrade

Just in case it doesn't show in the tech guide (I've recommended tech guides myself) - Q9650 if it's not a USFF.

Define "better".

A 2.4 GHz quad-core performs slower than 3.0 GHz single or dual core for most applications. Then again, if you do lots of compiling or video encoding then more cores is a plus.

"A 2.4 GHz quad-core performs slower than 3.0 GHz single or dual core for most applications."

This statement is wrong in its entirety.

You believe the MHz myth.

http://cpuboss.com/cpus/Intel-Pentium-4-630-vs-Intel-Core2-Duo-E8400

Single and Dual Core Processors based on Net-burst Architecture are MUCH Slower than the CORE 2 Duo or Core 2 Quad based units.

So a 3.8 Ghz  Prescott CPU with huge pipeline  is not as fast as a 2.0 Ghz Core2 Duo.  Even at the EXACT same Mhz Prescott is MUCH SLOWER.

The later Core Architecture Generations were Re Named Core I3  I5  I7

A DDR3 or DDR4   4th Gen Core I3 or newer Haswell CPU even at 1.6 Ghz is substantially faster than any of the older DDR and DDR2 Prescott CPU's at 3.8 Ghz.

They basically found that 4GHZ was the limit AND that even at that speed others were making processors with smaller piplines and larger L2 cache to beat them.

https://www.youtube.com/watch?v=TJN5GfZuVog

you can get helpful information about this from here.