Id go with XP for now, until its support stops next year. Then when support stops, you can get vista, it will be cheaper and running much more smoothly than it is now. And If you do do that then you wont really need to worry about much upgrading, and anyways if money is of no object then any uprades to the mobo and ram wont be too much of a problem...
But if I was you, Id get a good computer now, but not a super one, you can upgrade whenever you want, whats the point of getting a "super computer" Its like buying the most expensive car in the world, but you cant drive, or buyin a 16million dollar mansion for your cat to live in..
If I was you, I'd be putting that 700 a month into an ISA bank account and let the interest grow each month.. but thats just me..
Ooh, you have to get liquid cooling for the computer too, especially if you want it to show off... I would also probably insure it

Good luck!