Which Graphic Card suites most for gaming?

Solution
You don't state which i5 you have but based on this spec sheet I suspect it is a Lynnfield of some sort. Spec sheet has an i5-750 listed.
https://www.cnet.com/products/dell-precision-fixed-workstation-t1500-mt-core-i5-750-2-66-ghz-3-gb-250-gb/specs/

Also the system seems to come with a 350W PSU so that is limiting in some respect but I don't see it as an issue as the GPU choice will be limited by the CPU present.

A GTX 1050Ti I believe would be a good choice, matches CPU capability, low price, good availability and does not require a PCI-e power connector.
https://pcpartpicker.com/products/video-card/#w=50,80&sort=price&c=380&page=1
You don't state which i5 you have but based on this spec sheet I suspect it is a Lynnfield of some sort. Spec sheet has an i5-750 listed.
https://www.cnet.com/products/dell-precision-fixed-workstation-t1500-mt-core-i5-750-2-66-ghz-3-gb-250-gb/specs/

Also the system seems to come with a 350W PSU so that is limiting in some respect but I don't see it as an issue as the GPU choice will be limited by the CPU present.

A GTX 1050Ti I believe would be a good choice, matches CPU capability, low price, good availability and does not require a PCI-e power connector.
https://pcpartpicker.com/products/video-card/#w=50,80&sort=price&c=380&page=1
 
Solution