Dell XPS 8700 RAM upgrade or addition?

Hi All,

I'm a newbie here.

Just trying to figure out the difference and value btwn completely replacing my RAM by upgrading to a faster MHz OR just adding more RAM with the same (slower SPECS)

HEre's my PC:

Operating System
Windows 7 Professional 64-bit SP1
CPU
Intel Core i7 4770 @ 3.40GHz 58 °C
Haswell 22nm Technology
RAM
8.00GB Dual-Channel DDR3 @ 798MHz (11-11-11-28)
Motherboard
Dell Inc. 0KWVT8 (CPU 1) 28 °C
Graphics
FHX2300 (1920x1080@60Hz)
DELL 2009W (1680x1050@60Hz)
1024MB ATI AMD Radeon HD 7570 (Dell) 55 °C
Storage
223GB INTEL SSDSC2BW240A4 SCSI Disk Device (SSD) 31 °C
931GB Seagate ST1000DM ST1000DM003-1CH1 SCSI Disk Device (SATA) 31 °C
931GB Seagate ST1000DM ST1000DM003-1CH1 SCSI Disk Device (SATA) 31 °C

Here's my RAM:

RAM
Memory slots
Total memory slots 4
Used memory slots 2
Free memory slots 2
Memory
Type DDR3
Size 8192 MBytes
Channels # Dual
DRAM Frequency 798.1 MHz
CAS# Latency (CL) 11 clocks
RAS# to CAS# Delay (tRCD) 11 clocks
RAS# Precharge (tRP) 11 clocks
Cycle Time (tRAS) 28 clocks
Command Rate (CR) 1T
Physical Memory
Memory Usage 62 %
Total Physical 7.95 GB
Available Physical 2.99 GB
Total Virtual 32 GB
Available Virtual 25 GB
SPD
Number Of SPD Modules 2
Slot #1
Type DDR3
Size 4096 MBytes
Manufacturer Hyundai Electronics
Max Bandwidth PC3-12800 (800 MHz)
Part Number HMT351U6CFR8C-PB
Serial Number 274980024
Week/year 29 / 12
Timing table
Frequency CAS# Latency RAS# To CAS# RAS# Precharge tRAS tRC Voltage
JEDEC #1 457.1 MHz 6.0 6 6 16 22 1.500 V
JEDEC #2 533.3 MHz 7.0 7 7 19 26 1.500 V
JEDEC #3 609.5 MHz 8.0 8 8 22 30 1.500 V
JEDEC #4 685.7 MHz 9.0 9 9 24 33 1.500 V
JEDEC #5 761.9 MHz 10.0 10 10 27 37 1.500 V
JEDEC #6 800.0 MHz 11.0 11 11 28 39 1.500 V
Slot #2

Now, my understanding is that when replacing the most important Identifier is the Part Number HMT351U6CFR8C-PB. Just do a search on this part number and you should be able to find the exact match of RAM, right?

But, what's unclear to me is how do you find a REPLACEMENT RAM? Can I run DDR4? Can I run 1600MHz or faster in this motherboard? What is the timing?  Clock? This is where I get confused.

Also, will I even notice a speed increase?