You should be able to get the upgrade version of XP. It will ask for a prior version of Windows.
When I went from Win 98 to W2K, I did a clean install of W2K and in the beginning of the process it asked for a prior version of Windows I just slipped in the Win 98 disk for it to verify that I had a prior version of Windows and then it installed W2K.
Are you sure you want to go the build your own computer route?
There is nothing wrong with it, and a lot of people do it, but you have to ask yourself what is your time worth.
What if after you doing everything it won't boot up then what? Or there is a hardware conflict, there are so many little things that could go wrong, that aren't necessarily hard to fix, but time consuming.
If you are a profitable trader and this PC is for trading, how much is it going to cost you if you have problems with your computer and you can't trade.
I would check out a local PC shop, see what they have. Then go to Dell or another name brand manufacturer and compare.
I would be surprised that when you add up the components and your time that building it yourself isn't more expensive.
I am not trying to discourage you, just make sure you know what you are getting into.
best of luck,
Bruce Hawkins