That is a high end gfx card, you won't be able to use it at 100%, unless you consider an overall system upgrade later.
If I were you I'd go with a GTS450 and keep that 400W PSU. This card would cause a huge difference in the feling as well.
A GTS450 would be pretty much in the same category as your other system parts. Don't correlate to my setup, I have a much stonger cpu than video card. Go for factory speed modells.