Dell Optiplex Model DHP Power Supply and Graphics Card Upgrade

Hi all, i recently decide that i would be renewing my old dell optiplex by putting in a new graphics card. However, i forgot one small detail- i would need a new power supply as well. I am looking to acquire the ati radeon hd4650 agp 1gb graphics card, and i have determined that a 300W Power supply would be sufficient. This is my current power supply, and i have been looking at replacements such as this or this. However, the dimensions are a bit off, and I'm not entirely sure if all the connectors would work in the transition. Additionally, the location of the fan and the mounting position seem a bit off in relation to how it would fit in my computer. If anyone could give me some advice as to  whether these would work, or point me in the right direction, it would be greatly appreciated. Thanks!

Edit: the model is a GX270 slim form factor

Edit 2: Since the case is so thin, would i be able to leave it open and use a full sized agp graphics card, or would there be other conflicts with the case size?