Alienware X51 R2 graphics card upgrade

These are my specs...i think its and r2. i want to know what is the most powerful upgrade i can make to this systems gpu? I need to run Ark: survival and it runs like garbage with this setup lol.

• Processor

• CPU_Name: Intel(R) Core(TM) i5-4440 CPU @ 3.10GHz
• CPU_Manufacturer: GenuineIntel
• CPU_Caption: Intel64 Family 6 Model 60 Stepping 3
• CPU_Version:
• CPU_ProcessorId: BFEBFBFF000306C3
• CPU_CurrentClockSpeed: 775MHz
• CPU_AddressWidth: 64Bits
• CPU_DataWidth: 64Bits
• CPU_SocketDesignation: SOCKET 0

• Verified Hardware Device against Order: VARIABLEXYZ

• Verify Correct Processor Frequency

System Memory

BIOS Information

• BIOS_Manufacturer: Alienware

• BIOS_Name: BIOS Date: 03/17/14 15:31:05 Ver: 04.06.05

• BIOS_Version1: A06

• BIOS_Version2: ALWARE - 1072009

Motherboard Information

• MB_Manufacturer: Alienware

• MB_Product: 0PGRP5

• MB_SerialNumber: .FK1CV12.CN6970244H1514.

• MB_Version: A02

Video Adapter

• Video_Caption: NVIDIA GeForce GTX 745

• Video_AdapterRAM: 0MB

• Video_DriverVersion: 9.18.13.3266

Hard Drive(s)

• ST1000DM ST1000DM003-1CH1 SCSI Disk Device

• Disk_Model: ST1000DM ST1000DM003-1CH1 SCSI Disk Device
• Disk_Size: 1000 GB
• Disk_Partitions: 4
• Disk_TotalHeads: 255
• Disk_TotalCylinders: 121601
• Disk_TotalTracks: 31008255
• Disk_TracksPerCylinder: 255

Optical Drives / Other Storage

• HL-DT-ST DVD+-RW GA50N SCSI CdRom Device

OperatingSystem: MS Windows 7 Home Premium Edition

DirectXVersion: DirectX 11

IRQ Configuration

• 0 System timer

• 1 -empty-

• 2 -empty-

• 3 -empty-

• 4 -empty-

• 5 -empty-

• 6 -empty-

• 7 -empty-

• 8 System CMOS/real time clock

• 9 -empty-

• 10 -empty-

• 11 Intel(R) 8 Series/C220 Series SMBus Controller - 8C22

• 12 -empty-

• 13 Numeric data processor

• 14 -empty-

• 15 -empty-

• 16 Dell Wireless 1506 802.11b/g/n (2.4GHz)

• 16 Intel(R) 8 Series/C220 Series USB EHCI #2 - 8C2D

• 17 High Definition Audio Controller

• 22 High Definition Audio Controller

• 23 Intel(R) 8 Series/C220 Series USB EHCI #1 - 8C26

• 190 Microsoft ACPI-Compliant System

Miscellaneous

Final Checks

Installed Devices Drivers

• Realtek High Definition Audio, 2-19-2013, 6.0.1.6844

• Microsoft Streaming Tee/Sink-to-Sink Converter, 6-21-2006, 6.1.7600.16385

• Generic volume shadow copy, 6-21-2006, 6.1.7600.16385

• HL-DT-ST DVD+-RW GA50N SCSI CdRom Device, 6-21-2006, 6.1.7601.17514

• Intel(R) Display Audio, 11-27-2013, 6.16.0.3131

• Microsoft Streaming Tee/Sink-to-Sink Converter, 6-21-2006, 6.1.7600.16385

• Generic volume shadow copy, 6-21-2006, 6.1.7600.16385

• Intel(R) Core(TM) i5-4440 CPU @ 3.10GHz, 6-21-2006, 6.1.7600.16385

• ST1000DM ST1000DM003-1CH1 SCSI Disk Device, 6-21-2006, 6.1.7600.16385

• Generic volume shadow copy, 6-21-2006, 6.1.7600.16385

• Intel(R) Desktop/Workstation/Server Express Chipset SATA RAID Controller, 8-28-2013, 12.8.2.1000

• Generic volume shadow copy, 6-21-2006, 6.1.7600.16385

• Intel(R) Core(TM) i5-4440 CPU @ 3.10GHz, 6-21-2006, 6.1.7600.16385

• Generic volume shadow copy, 6-21-2006, 6.1.7600.16385

• Intel(R) Core(TM) i5-4440 CPU @ 3.10GHz, 6-21-2006, 6.1.7600.16385

• Generic volume shadow copy, 6-21-2006, 6.1.7600.16385

• Realtek PCIe GBE Family Controller, 12-26-2012, 7.67.1226.2012

• Microsoft Streaming Service Proxy, 6-21-2006, 6.1.7600.16385

• Dell Wireless 1506 802.11b/g/n (2.4GHz), 11-30-2012, 9.2.0.514

• Intel(R) Core(TM) i5-4440 CPU @ 3.10GHz, 6-21-2006, 6.1.7600.16385

• NVIDIA High Definition Audio, 11-28-2013, 1.3.30.1

• Intel(R) HD Graphics 4600, 1-29-2014, 10.18.10.3412