I still using my original HDD since i bought my XPS 8700 from May 2014.
I just realized the motherboard can upgrade SDD.
I'm wondering does it worth it. I know it help speed up on loading time and gaming can increase 2-5 FPS difference..
A few reasons. First of all your original HDD is approaching 6 years old and mechanical drives will fail, it's just a matter of when. At 6 it may be nearing that time. SSDs are much more reliable and about 5x faster. That means loading times for programs, booting times, and faster read times such as for games.
buy a used $20 SSD try it, if not like, sell it,
hint, nobody ever sold SSD ever because they stopped loving it. nobody, size yes, speed no.
x300s 128GB used, try one, or even a faster one. seen on benchmarks dot com.
the first thing you see, click control panel and it is near instant , not 10seconds like the rust drive. now.
I bought an inexpensive offline clone device (under $40), then bought a 1TB SSD (Crucial and Samsung) and cloned my current HDD that came with the computer. Then replaced the HDD and booted right up. I ended up having a problem, though, because I had also upgraded to a 4790K. Now it boots so fast that when I need to get into the BIOS (for example to change to OC1 or OC2) then I have to immediately start pressing the <F2> repeatedly and then I usually have to shut down five or so times before I pop into the BIOS. Thought it was pretty funny that it is now "too" fast.
You will love the SSD. I tried several other types of copy software and the offline clone was significantly better.
To properly mount the SSD you will need a 2.5" to 3.5" adapter bracket. Something like this should work: https://www.amazon.com/Sabrent-Internal-Drive-Mounting-BK-HDDH/dp/B00G57BN1M/ref=sr_1_10?crid=22DWK6...
"do i really need to mount it? that is the only options? And i only need to get the Mounting Bracket?"
I don't personally know the XPS 8700, but I'm assuming that it doesn't have a dedicated 2.5" Slot!
If so, whether you are trying to fit a 2.5" HDD or a 2.5" SSD, you will need to have a Bracket to enable it to physically locate in an existing 3.5" Slot.
Personally I would suggest that the Bracket suggested by @Vic384 would have been very suitable - however, there are plenty of alternatives on the Market!
There are other options although not as recommended. You could use Velco tape or zip ties. Obviously with the mounting bracket you will need some screws to hold the mounting bracket to the drive cage and the SSD to the mounting bracket. Some mounting brackets come with screws, #6-32x1/4" and M3x5. You will also need a SATA III data cable and unless they are all used in your computer a SATA power connection.