Graphic card for a XPS/Dimension 400/9150

Your system uses PCI Express X16 video cards so you are in luck.  It does not use AGP.  PCIE came out in 2004.  Your 375 watt power supply is enough for many cards today as they are more power efficient than the old ones.  If you just want a card that is similar, but a bit better than what you have now, you can just get a 9500 GT as shown HERE.  However, there are certainly much better cards today for about the same cost like the ATI HD 5670 HERE.  You can also still find some 9800 GT's for less than $100.  HERE is one.  

There are really a lot of choices for you.  The 9500 GT will have no problem doing what you want, but there are a lot of newer cards out there to choose from.  Newer Nvidia cards between the performance of the 9500 GT and 9800 GT are the GT 430 and GT 440 so you can search for those too.  The HD 5670 and the 9800 GT probably have about the same performance, but the HD 5670 is newer technology and is a Direct X11 card versus the older 9800 GT that is Direct X10 only.  Be careful on the GT 430 and don't choose one that is only 64-bit.  Make sure it is a 128-bit card.  All of the other should be 128-bit.  Some do have the better GDDR5 memory also versus GDDR3, but they generally cost more and this is not as critical for you given what you'll be doing.  

For less than $100, I would probably choose the HD 6670 with GDDR5 memory, but that's me.  It would not be necessary in your circumstances.  You could also get the HD 6750 as your high-end choice since it can be found at or just above the $100 mark.  For the price though, you should be more than happy with the HD 5670 and it has the newer technology over the 9500 GT and 9800 GT.