Dimension 4600 & Nvidia Personal Cinema FX5200

I have a Dimension 4600 that presently has 1 gig of RAM and two 120gb hard drives. I am running a DVD-ROM in the top and a DVD Burner as a second unit. I am using this machine for video purposes only.  I presently have an older ATI all in wonder 128mb card in this machine but it is having problems and needs to be replaced.

I would like to put an Nvidia Personal Cinema FX5200 agp card in but I'm not sure about power since my Dimension has a 250WATT psu in it and the video card says a min of 300 watts.  Would I be better off to wait and order a new psu before installing the video card?  I'm not really interested in all the bells and whistles on this card right now but am more interested in just capturing video from VHS tapes to the hard drive so I can't see where the big power need is. However, at a later date I intend to use the DVR capabilities and the TVinput, etc of this card.  If there is a need for a new PSU, does anyone have any suggestions on a good one that would be dependable?  Thanks