Well looking at the AMD site it states a minimum 400W PSU for basically every low end video card, even though here ---> http://www.techpowerup.com/reviews/Sapphire/HD_5750/28.... a 5750 should only consume 86W at most leaving your Dads system with about 150W for the rest of it. Personally I think a you could get away with a 5450 and it would be fine. But if you go by what the manufacture says it will not work. One of the bummers of buying a HP/Dell etc, they weren't meant for upgrading bits and pieces within them.
1) You MAY wish to consider an HD6450 card instead, since it's newer and the price difference is minor.
2) If you has 32-bit Windows and 4GB of physical RAM installed, I recommend a 512MB version. If he only has 2GB of physical RAM, then a 1GB (1024MB) version is installed.
WHY? Windows 32-bit can only address 4GB of memory total. This includes the System RAM (the DDR2 or DDR3 you installed on the motherboard) as well as the Video RAM and some minor other stuff. So if you have 4GB of physical RAM you'll see maybe 3.4GB with a 512MB card and about 2.9GB with a 1GB graphics card.
*Software decoding recommended unless your CPU is too old (most dual-cores or better are fast enough). Hardware decoding uses less power, but make sure to disable any Advanced Video settings in the AMD Control Panel (i.e. Flesh Tones etc).