An AGP Pro slot accepts ordinary AGP cards. You do not have to
buy an expensive card just to fit the slot. The AGP pro slot accepts
expensive Pro cards and it also accepts ordinary AGP cards.
The Pro slot provides extra power pins, as a Pro card usually is quite
large and can have a lot of memory chips on it. See page 14 of this
doc, to see the extra power pins. An ordinary video card, like my
ATI 9800Pro, can also get its power from a disk drive connector -
that was done because most motherboards have non-Pro slots, so ATI
or Nvidia cannot rely on finding a Pro slot when they need additional
power. In other words, the Pro slot, while providing the option to
support more power hungry cards, is not the only mechanism for giving
that power - there is also the "disk drive cable" method. I use
a 9800Pro (which has an ordinary AGP connector) on my P4C800-E.
http://www.motherboards.org/files/techspecs/apro_r11a.pdf
Here is an example of a card using the AGP Pro style connector.
This card is long enough, that it will go right across your
motherboard. Cards like this are typically purchased by mechanical
engineers needing good OpenGL performance for CAD work.
http://www.3dlabs.com/products/product.asp?prod=273
If you are a gamer, anything listed in this very recent article,
will be suitable for use in your motherboard:
http://graphics.tomshardware.com/graphic/20050705/index.html
HTH,
Paul