Buying New PC, are these specs OK?

Well, this is the system I want, I am trying to play all flight sims, I just posted in this forum, becuase the members seem by far the most helpful, and it is also the most demanding game, (Along with FSX with mega airport and AI)

http://www.chillblast.com/pconf.php?productid=17826

If you scroll down the page, I have selected the Most powerful processor, and the 3072mb Nvidia.

However, I have unchecked the the drive, to the least powerful (I don't need to read/write BluRay) I have also unchecked the OS because I have the install disks at home (for XP and Win7) Would it be adviseable to have them install the OS for me? (The total cost is 1500 pounds)

Which brings me onto my penultimate question... I play all flight sims, and some of them don't work on windows 7, so would CoD run anywhere near as well on XP?

And my ultimate question, could I get an equally powerful computer for less money? (I didn't ignore the advice of the people telling me to get i5, but the option is simply not on the sight)

Oh and how many FPS would this system give me, if all settings were max?

Thanks in advance!

Jack