Nvidia Geforce 8400GS and Dell Optiplex GX260 problems

I bought an Nvidia Geforce 8400GS on newegg (http://www.newegg.com/Product/Product.aspx?Item=N82E16814133245&cm_re=PNY_8400GS-_-14-133-245-_-Product), but it will not work in my Dell Optiplex GX260. It loads past the bios screen just fine, but it gets a black screen just before the windows logo comes up. I can not even boot into safe mode because it blacks out just after the bios loads. I know the card itself works, because it worked in my brothers PC. It can't be the PCI slot because I have tried all four slots, and another card I have works in each of them.
I have upgraded the PSU to the recommended amount (300W), and I still have had no avail.

I am running Windows XP on a Dell Optiplex GX260 with a P4 3.06GHZ and 1GB of RAM.

please help me out with this, I do not want to be out $50

 

You might still be able to boot into Safe Mode. Try to enter the Safe Mode prompt by pressing F8 after POST.

If the prompt comes up, then congrats. Boot into Safe Mode and remove your old display drivers. If nothing comes up, then...

I dunno, your fault for buying a Dell. :

 

I'll try that again I suppose...

I didn't buy it, it was given to me

Update:
Ok so... it only seems to work when I have another card plugged in to boot with, and then I can use the Geforce 8400GS as a secondary monitor... the only problem is, I currently only have one monitor with a power cable. It seems to work fine after the PC has booted up, so I will have to find that other plug so I can be able to actually use the thing the right way

 

I tried that, it still wasn't working.
But I once I find the cord for my other monitor, things should be alright with it