X51 R2 system psu is up to 330W.  Practically speaking best gpu supported is GTX 1050Ti (300W psu requirement)

get a blower style, e.g., founder edition if possible (better cooling for X51 chassis).

https://gpu.userbenchmark.com/Nvidia-GTX-1050-Ti/Rating/3649

https://www.dell.com/community/Alienware-General-Read-Only/Alienware-x51-r2-Power-Supply-Upgrade/td-p/5509537

The gpu that best matches your i7-4770 cpu in speed is

GTX 1060 (6 gb) (minimum system psu 400W)

or

RX 580 (8 gb) (minimum 500W psu)