I would definitely go with the P5AD2.
The ASUS can overclock quite nicely. Check this
You should also get the PCI-E card rather than the 9800. I haven't seen any benchmarks comparing the new series of video cards from ATi and nVidea (x600, x700, and 6600). So, I don't know which one to recommend for you
why will d 9800pro 128mb perform better than the 256mb version? shouldn't it perform better?
It doesn't seem logical, but it somehow benchmarks higher