I would buy that system with the ATI card and try it out.  If you find that the ATI card doesn't work well for you, then spend another $100-$200 for a NVidia card.  You can always sell the ATI card on EBay or through your local adds.

In my personal experience, I found no difference between the ATI and NVidia graphics.  Mind you, I used older ATI cards.  I 'hear' that Nvidia has a better OpenGL implementation, but I cannot say from my own experience.  For my last graphics box, I spec'd out a Nvidia GTS250 - works very well.

Be aware that paying more for a higher end card (ATI or NVidia) is not worth it as VUE will not use the power.

The other option you have for that price is to go into a computer shop and get an estimate for having a system custom built to what you want within your cost and see where that will get you.  This is ussually the best way to go - no compromises that someone else makes on your behalf.