IMO drop the Phys-x card unless your going SLI (from what I have read not many games are supported fully anyway and what games are supported if you do run it requires alot of GFX power as it actually lower's your FPS by putting out more for your card to process) its realism vs smoothness.... but this is only from what I have read (I could be totally wrong).
Many people here are going to say go with a 8800GTX and OC it rather than an ULTRA... but I say if an ULTRA is what you want, get an ULTRA.
I would recommend dropping the PSU a bit as 1000w isn't really required from what I have been informed for a system like that, but seen as it comes with your case... meh.
I will stop here as I am no expert by any means, but I will say make sure the RAM you are choosing is on Asus' QVL list as I have been looking at the same MB and have already had enough troubles finding the right RAM for it.
Don't bother with a SLI board unless you plan on it, upgrading to another single card solution, to this day, seems the most logical way to go. By the time a second Ultra falls in price for you to slot it in, the next gen cards blow both of yours away. But, it is nifty to have, so I'm not arguing it really.
Go for a GTX, the Ultra is just a waste of cash on a first gen DX10 card.
Don't bother with the Physyx card. Programmers should use CPU cores that are mainly Idle to handle such tasks. I hope that Physyx never takes "hold" as a standard. Having to worry about the next CPU and Videocard to upgrade is expensive enough without giving a new BS company a chance to rip us off when our systems have multi-core chips in place. What's next, a Hope and Wishful Thinking card we need to supplement our multi-core chips workload? Meh...
You don't need any more than 800W for your comp, try for 700W even, save some $$$. Just pay attention to the amps on your 12V rail and get a Thermaltake or Seasonic PSU, pref. Seasonic. Make sure it is SLI certified, more amps and a second PCI-E power cord will be waiting should you go SLI at any point without having to mess with adapters, or be lacking amps.
Is your copy of XP OEM that you used on another comp? If it is, you need to buy another copy of XP then, just so you don't burn your key. At this point though, I'd just get a Vistax64 OEM, SP1 should be out soon. But if you have the money, go XP and hang on to it FOR AS LONG AS POSSIBLE. Then get Vistax64 upgrade, and use the clean install trick.
I had an OEM copy on a computer and I upgraded the RAM, then the mobo, then the GFX card. Then I built an entirely new computer and put it on there. Everytime i was able to activate it over the internet without problems. I think microsoft has relaxed their policys quite a bit