Vostro 270s, can I add a SSD?

The tricky part is that the 270s only has 2 SATA ports and they are connected to the HDD and the Optical drive. You could either purchase a PCIe adapter card for an NVME drive, or a SATA adapter card for a SATA SSD. Or you could disable the optical drive (pull the SATA and power cable) and use them on the SSD and 2 sided tape the SSD to the drive cage. Not very elegant but it would work. And it should be compatible as Vostro 270s system were available with a SATA SSD. Of course you could always pull out the hard drive, buy a larger SSD , and install it in the system. Then clean install Windows. It all depends how much work you want to do.

Yes this will work fine.  I also recommend getting a SATA Wire to allow cloning of the internal drive to the ssd.  PCI-E to NVME is NOT an option for a vostro 270.

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_vostro_desktop/vostro-270s_owner%27s%20manual_en-us.pdf

You will need a Silverstone SDP09 adapter to mount the 2.5 inch drive.

https://www.bestbuy.com/site/apricorn-data-transfer-cable-gray/3459621.p?skuId=3459621

https://www.amazon.com/SilverStone-SDP09-2-5-Inch-3-5-Inch-Hot-Swappable/dp/B0049MPQDG/

Thanks very much for your input. I have a look at those adapter cards and they are not cheap. My goal is to improve desktop performance. I have read about HD and SSD combination which most frequently used data will be on an SSD and less frequently used data will be on an HD. Adding an SSD alone won't achieve that data storage arrangement, I guess.   

You can remove the optical Drive and put the 3.5 inch drive there.

https://www.amazon.com/iDsonix-Tool-Free-Internal-Mounting-Extraction/dp/B00L3W4G1I/

PCI-E to SSD is NOT an option EVER.

You do not have UEFI bios nor do you have X4 PCI-E slot to put such a card in.

 If you still want optical drive you can put that into USB2 enclosure.

https://www.amazon.com/OWC-Mercury-Optical-External-Enclosure/dp/B06XRCCV44/

Thanks very much for the info. 

I have watched some related videos on youtube. A few of them also mention that clone an HD to a newly installed SSD. Afterward, how does the OS decide where to write new data, HD or SSD?

New Data such as Games Etc should go to the Hard Drive whoch you assign the drive letter D to making an external optical drive E.

World of Warcraft and Fallout 76 for example is 60 GIGS of Data for each game and routinely updated with 15 gigs or more of "patches".   SSD's have a maximum number of writes and they are dead.   Installing all of the blizzard Games for example uses over 250 Gigs of data.  That does not include patches and updates etc.

An SSD does not increase the speed of most games.  It does increase the boot time from 1 minute or more to 25 seconds or less.