VisionTek R7 260x graphics card issues

Hi Folks,

 I was wondering if anyone can shed some light to why I’m experiencing this problem. I purchased a VisionTek Radeon R7 260x graphics card (2 gb) from Dell 2 years ago and I tried to install it twice but with no luck. I wanted to upgrade the current graphics card with a better one, hence why bought the one I mention based on the recommendations from the Dell sale agent. Now, the issue that I experience is, based on the system requirements below for the new graphics card,

 Minimum 4GB of system memory.
500W (or greater) power supply with one 75W 6-pin PCI Express power connector recommended.

www.newegg.com/.../Product.aspx

I needed to upgrade the 460W power supply unit that came with my XPS 8300 in which I did, going to a 550W VisionTek power supply.

When I first installed the new graphics card, everything was connected as it should be and when I turned on my PC, all I got was a black screen, as if the monitor was trying to display or look for a signal. I had rechecked everything to ensure all was connected and tried again but no luck. I took my PC to a friend just to see if he knew and no luck. We installed the new graphics card on his PC and it worked there. Hmmmm, I wondered??? So we tried once again and again with no luck. He suggested that perhaps my the new graphics card wasn’t compatible with the motherboard. So at this point, I didn’t bother in returning it for a refund because I came around to installing the new card 4 months after I purchased it. I know, but I was busy with life and work. Anyway, 2 days ago, I bought more memory and I figure I try again, installing the new graphics card thinking that the PC lacked memory the first time around. Despite adding more memory, I still had no luck. I tried connecting it with the HDMI cable and DVI to VGA cable and no luck.

Below are my specs for your reference:

Before:
8 GB memory
460W
ATI AMD Radeon HD 6450

Currently:
12 GB memory
550W
ATI AMD Radeon HD 6450 replacing it with the VisionTek Radeon R7 260x (2GB) graphics card
BIOS version = A06, 10/17/2011 

Therefore, I’m asking for feedback or a solution that could help me in this problem. Better yet, if there is a much better graphics card, say 4GB, that would work on my XPS 8300, advice would be great. I figure that once you install a new graphic card, it should be easy. Take out old one, put in new one, turn on PC, start installing the drivers for new graphics card and start enjoying playing games. Sorry for the long discussion. Thank you.