Essentially, the 'minimum power supply' requirements for graphics cards are massively overstated, so that if you use a crappy brand unit that can't put out anywhere near the wattage it says it can, you won't experience issues. In reality, the GTS250 will use about 100-120W at full load, and the rest of the system usually no more than about 80-90. With a Pentium D it'll be a bit more than that, but suffice to say, a good 350W PSU could run this system, let alone a 450.
