So as the title says I'm fairly new to buying PCs as I haven't myself had a new one in ten years. This is my first, and rather than build it piece by piece I went with the alienware aurora with the following hardware:
Okay, first of all there's something very important to note here. The GTX 260 in your computer is a Core 192 version, not the newer 216 version. Regardless of Nvidia saying that they should SLi fine together, people usually have problems if they try and mix them. Unfortunately, the card by itself will also provide worse performance so I don't think you'll be getting too great of frame rates in Crysis on High/Very High (in 1920x1080) until you add another card.
Get the bigger power supply if you're going to overclock and have two video cards. The GTX 260 can draw up to 180 watts by itself. I have almost the same system and I went with a 700+ psu to compensate for max load and aging.
If you only ever have one video card, then the smaller PSU should be fine. It's unlikely any future single GPU will draw much more power than the 260 core 192.
Your overclock will likely be the deal breaker. Full load on the whole system could push it over the edge. Either you'll need the bigger PSU, or you'll have to hold off on overclocking until you get a more energy efficient video card.
(gtx260) 192 or 216 core will work together. I would recommend getting another gtx260 for sli. Im running 2 oc gtx 260 sli and they run great and can pretty much max out crysis at 1920x1080. If your psu is a quality psu then you can overclock it. Im running a q6600 @ 3.2ghz 6gig memory and 2 oc gtx 260 sli on a antec earthwatt 500.