Inspiron 3668, GPU upgrade?

Thank you for your answer, I see. So can I use the Amd Ryzen 5 3600 with this inspiron 3668 ? By the way the asus graphic card Nvidia Ge force Gt 730 is compatible with the motherboard ? 

Oh no. Intel and AMD are like water and fire incompatible. Definitely no Ryzen cpu for intel motherboard in 3668.  Gt 730 gddr5 is a card worth considering and definitely compatible. The older gt730 gddr3 also works but is inferior.

Intel i7-7700 and i7-7700k are two validated cpu for 3668.  Your pc is already configured w i5-7400.   If you use i7-7700k you have got to upgrade cpu cooler above Dell pancake cooler and try add VRM heatsink if possible. Inspiron is main stream line pc not high end compared to XPS or Alienware.

but if your plan is to build a budget gaming pc then do not spend on cpu upgrade now but focus on gpu and psu needed.  
The ASUS gt730 you were looking at is not good for gaming although it has 4 x hdmi ports.  If all of your three monitors have hdmi only then that may be a good choice for display only (not gaming).  The stock psu is enough for gt730.

https://www.userbenchmark.com/PCBuilder?searchTerm=Inspiron%203668%20i7-7700k
https://www.userbenchmark.com/System/Dell-Inspiron-3668/50388

No it is not for gaming, just for work programming something fast usting the browser also but I was thinking to spend less than 200 $ for an Intel I have small budget at least 400$ for everything If possible an Intel, gt730, update ram 2x8gb and may be an update power supply 

Thanks for you advice. do you know any good budget ram ? Because everyone say corsair and do I need to buy a cooler fan for the core ? 

Dell stock 240w psu is most likely okay for ASUS gt730 gddr5 with a tdp of 38w.

basic psu calculator estimates Load Wattage: 215 W
Recommended PSU Wattage: 265 W

load wattage is the max system may consume for typical real world applications.

the new Asus GeForce GT 730 caters towards professionals that are looking for a graphics card to enable multi-monitor productivity, which does give it at least an excuse of relevance even today. The graphics card comes equipped with four native HDMI 1.4b ports with HDCP 2.2 support and drives monitors with resolutions up to 4K (3840 x 2160).

I think I'm going to keep the I5 7400 is still good 

 and add gt730 with a ram 2x8gb there is good cheaper than corsair ? And it think I will need to udapde a new power supply too? By they way do I need to adapter fan for the the core ?

User validated ram

Use the ram maker model number to look on eBay for deals. Crucial may have good deal too.

3668 (like 3650 which supports intel 6th gen cpu) has a hdd cage therefore can support low profile cooler only.

This one has a copper cold plate and 120 mm blue led.  

• Recommended for TDP 95W
• 60mm Height For limited form factor cases (use ruler to measure 3668 cooler clearance before you buy)

refer to this user report Reeves Brontes cooler low profile (out of stock).

• Ok thank you for all those informations 

using stock 240w psu? is this for real? or too risk?