power supplies are crazy beasts number one rule is always go larger. The number one question is always how large is large.
One of the best gauges you can have for a power supply requirement is the video card specs.
The GTX 275 EVGA (lowest grade) lists the minimum power as 550 Watt (minimum recommended power supply with +12 volts current rating of 40 Amps). Two available 6-pin PCI-E power dongles.
The GTX 285 EVGA (lowest grade) lists the minimum power as the same. The EVGA GTX 295 jumps up to 680 Watts and 46 amps.
These are the minimum specs the problem becomes the technical writers / sales department are never on the same page as product development. Never trust minimum specs. Use FSX as a gauge of how far off they can be. Minimum specs are written for Video cards based on what the card needs. They do not take into account what else is on the system. Start adding optional extras like CPU, Memory, Motherboards, flight controllers and the minimum specs just went way up.
The last thing you need to remeber about PSUs is they degrade with use. Antec recommends adding 20% if you plan on keeping your PSU for more than a year or 25 to 30% if you run it 24/7 usage for 1+ years.
See their calculator which I believe you can trust.
www.antec.outervision.com/PSUEngine
Based on what you posted their calculator generates the recomended wattage at 668 watts. That number is with out Capacitor Aging (degrading). If you plug the number of 20% in because you plan on keeping the PSU more than a year you need 802 Watt of power.
And we have not even begun to discuss that crazy little number in the small print on the Video card specs. The PSU must produce the recomended amperage on the 12 volt rail or this little theory call Ohms Law of relativity kicks in. When that happens before you know it

the whole thing may go south. The very least of what will happen is your video card will not function to the desired results.
Hope this helps but in my opinion if you plan on purchasing the PSU now to add it to a new build later you need to get in the 1000 wat range.
BTW I will back up Antec's claim to add more for Capacitor Aging. My PSU is 3 years old now and can no longer run every thing I have plugged into it. I bought a 500 watt power supply and running it every day has taken its toll.
Good luck but one of the last things you want to go cheap on is the PSU. It drives everything now and every thing you add later.