Dell G3 15 Setup and Specifications manual
says this about your memory:
Therefore to answer the questions:
Question 1 - will adding another 8GB memory of same speed would change it to Duel Channel?
The processor, 8th Generation Intel Core i5/i7, is recent enough to have
a memory controller that supports the multi-channel architecture,
and the manual of the motherboard specifies as above that it
supports this architecture.
These are two of the three conditions required for dual-memory.
The third condition is that the two memory modules must be completely
identical. This means not only of the same speed, but completely
identical in all attributes (of which there are many).
This is why most memory vendors also sell kits of identical modules
for activating the dual-channel mode.
Question 2 - if my memory is running at the half the speed
One should not confuse here speed and bandwidth.
Your memory will run at the very same speed under both dual and single modes.
Under dual-mode, the bandwidth is doubled, but read on for performance
This is what Wikipedia
Multi-channel memory architecture says about performance:
Theoretically, dual-channel configurations double the memory bandwidth
when compared to single-channel configurations. This should not be
confused with double data rate (DDR) memory, which doubles the usage
of DRAM bus by transferring data both on the rising and falling edges
of the memory bus clock signals.
Tom's Hardware found little significant difference between
single-channel and dual-channel configurations in synthetic and gaming
benchmarks (using a "modern (2007)" system setup). In its tests, dual
channel gave at best a 5% speed increase in memory-intensive tasks.
Another comparison by Laptop logic resulted in a similar conclusion
for integrated graphics. The test results published by Tom's Hardware
had a discrete graphics comparison.
Another benchmark performed by TweakTown, using SiSoftware Sandra,
measured around 70% increase in performance of a quadruple-channel
configuration, when compared to a dual-channel configuration. Other
tests performed by TweakTown on the same subject showed no significant
differences in performance, leading to a conclusion that not all
benchmark software is up to the task of exploiting increased
parallelism offered by the multi-channel memory configurations.
This is because each 64-bit channel connects to one RAM module.
To perform effective 128-bit accesses, the software must simultaneously
address the two modules, which only specifically-written software can do.
Most applications and operating systems are not built with such
difficult to program optimizations.
As CPUs become more and more multi-core, performance will increase
with the parallel execution of programs on more than one core,
where the bandwidth may possibly be more fully exploited.
However, the cases when several RAM-intensive programs do run in parallel
at the same time are usually few enough.