I would not put the regular PCI video card in the PCI express board for many reasons.
For the most part, ASUS does not play friendly with many other video cards and vice versa.
For example, I bought the ASUS AX 800Pro/TVD 256mb video card for, at the time, my Shuttle mother board.
Man, I had all kinds of problems; video games would just stop, white screens, overheating, and you name it.
I called ATI first and they clued me in that ASUS does not like Shuttle boards and the video cards get hot.
The next call was to ASUS and they told me exactly what would work with the video card. So in a sense, I built my computer around my video card.
I think you should do the same but like the guy a ATI told me, research before you buy. Yeah right...after I bought the $600 vid card he tells me!
If you have a ASUS PCI Express mother board then I would wait until I got the ASUS XT800XTPro Platinum PCI Express card before doing anything with the board.
PCI Express is so new, fast, and expensive; I would take my time on this one.
Also, before you do any building I would think about how you are going to cool your computer.
I know this sounds boring but cooling is so important especially to gamers. When you have so much invested cooling seems like the last issue.
It should be the first. If you do create a PCI Express computer don't rely on the manufacture to provide adequate cooling. You will have too.
Let us know,
Your Hardware Guide.