dontlistentome :
It's about the power supply. If you have an HD 78xx series card in there, that means you have the pci-ex slot for the card. You need to make sure your power supply has the necessary 6 or 8 pin pci-ex connector or connectors that the 760 needs. Since you did not provide the exact model of either card, I can't tell you what you're using now and what the 760 needs.
Here's My Specs:
(My Specifications On My Alienware Aurora R3)
A.) Operating System
Windows 7 Home Premium 64-bit SP1
B.) CPU
Intel Core i7 920 @ 2.67GHz
Bloomfield 45nm Technology
C.) RAM
24.0GB Triple-Channel DDR3 @ 534MHz (7-7-7-20)
D.) Motherboard
Alienware 0H869M (CPU 1)
E.) Graphics
VMM26F10I (1920x1200@60Hz)
1024MB ATI AMD Radeon HD 7800 Series (XFX Pine Group)
F.) Storage
SSD# 1.) 476GB SanDisk SD8SB8U512G1122 ATA Device (SSD)
HHD# 2.) 0B SAMSUNG HD204UI ATA Device (SATA)
HHD# 3.) 0B SAMSUNG HD204UI ATA Device (SATA)
HHD# 4.) 4657GB Seagate ST5000DM000-1FK178 ATA Device (SATA)
G.) Optical Drives
DVD# 1.) HP DVD-RAM GH60L ATA Device
DVD# 2.) PLDS DVD+-RW DH-24AAS ATA Device
H.) Audio
Realtek High Definition Audio