You have an M.2 slot but it is SATA not NVMe. That being said the SATA M.2 SSD is still around 3-4 times faster than an HDD. This YouTube video offers some insight into NVMe vs SATA SSD.

If I were you I'd get a PCIE card that has an M.2 NVMe port on it. There are even adapters that have 2x M.2 NVMe ports on them, but they can be more expensive than buying 2x PCIE adapter cards. Find one whose brand or company has been around for years, like StarTech's PEX4M2E1. Otherwise you get what you pay for.

@DynamicDoyle As I just mentioned to @JOcean I am confused which M.2 port I have on my motherboard as the label mentions M.2 SATA1 while it looks like an NVMe compatible port. As SSDs sold online  are "non returnable" I don't want to take a chance. Would help if you can suggest a freeware appln which can confirm which M.2 port my PC has.

@JOcean I am confused as the M.2 port on my motherboard mentions M.2 SATA1 but the port itself looks like one meant for NVMe (with a single notch on the right, with what's called 'M' key edge connector). As it's a proprietary motherboard of Dell I am unable to verify it by looking it up on Google search. It would help a lot if I can find a freeware s/w which detects and displays ports on motherboard. 

From the Dell support page tech specs for the 3470 found here, the following:

Storage specifications

@JOcean- Thanks for the additional info. Meanwhile, I checked again with Dell TS who said I can only have a 128Gb M.2 SATA SSD in my PC if I retain the HDD. If I remove the HDD then I can install an M.2 SATA SSD of up to 512 Gb. Considering the high price of SSDs (possibly due to global chip shortage) I may have to defer my purchase. Hope this one from Samsung would be compatible - Amazon.in: Buy Samsung 860 EVO 500GB SATA M.2 (2280) Internal Solid State Drive (SSD) (MZ-N6E500) Online at Low Prices in India | Samsung Reviews & Ratings

Yes that Samsung would be compatible and this one would be compatible as well. The reason that Dell TS told you a maximum of 128GB is that is the largest Dell tested with the 3470 but not the largest that is actually compatible.

@JOcean Thanks for the response. I don't need a 1Tb storage as keep my personal data on cloud storage. So, are you saying I could buy & install the 500Gb Samsung SSD (link in previous post) AND retain my current 1Tb HDD too (as Dell TS said 128 Gb SSD is max I can use WITH my HDD)? 

@JOcean Thanks. I intend using an SSD only for OS, but if I manage to get a 500 Gb one I may use it for personal data storage as well, and remove the 1 Tb HDD. I wish I could use an M.2 NVMe SSD which is much faster than a M.2 SATA SSD (though I know that even a SATA SSD would be 3-4 times faster than my HDD).

Checked physically and noticed that my Vostro 3470 Desktop PC motherboard has an M.2 SATA 2280 with 'M' key edge connector only (no 'B' key edge connector). So, can't use M.2 SATA 2280 SSD from links shared in earlier posts. Only NVMe SSDs seem to come with 'M' key edge connector

    Hey Ramesh, I have owned a Dell Vostro 3470 since 11/19. A month later I installed a Western 

Digital WDS500G2B0B SATA M.2 drive and doubled up the memory to 16 GB. The startup took

around 50 seconds with the original HDD. The sata ssd dropped it to 15 seconds. I  cloned the HDD 

to ssd and kept the Hdd as extra storage. The difference was amazing. I am now considering adding 

an NVME like a previous answerer suggested. You probably should just add it by way of a PCIE adapter

and skip the SATA. Why go 3.5 times faster when you can go 22 times faster? 

ps the m.2 slot on your mobo is bm key sata ready. 

Bro It said i can get up to 2TB HDD but now I got a 3TB HDD and nothing happen

Can I use both 256gb m2 sata disk and a 3tb hdd?

      Nguyen, you should be able to use both drives M.2 ssd and the hdd at the same time. I'm not sure what you mean that nothing happened with the 3TB drive. If you cloned your boot drive to the ssd you can change the boot order in the bios to run the ssd as a boot drive.