Any AGP interface will work, provided you avoid any cards that need a dedicated power feed, as your PSU may not have a spare lead of the correct fitting. So nothing top of the range, but anything else would be fine. Don't worry about the speed of the AGP interface - 2x, 4x or 8x - it'll run at the speed of the slowest side of the interface.
We can't answer that without knowing the max res of each of those chipsets. The X800 almost certainly would, and the 9800 probably would. You'll have to check for yourself if you actually want a Crucial graphics card, as the site has a distinct lack of information. The ATI website would be another source of info, since these chips happen to be ATIs.
Would you not consider another brand? They are mostly pretty good and are made to reference designs. Don't necessarily restrict yourself to an ATI chip either. What's wrong with nVidia? If you want ATI, a Sapphire card would be good, or perhaps XFX for nVidia. Have a browse at Dabs; it has a huge range. Remember any AGP interface will be fine as long as the chipset meets your res needs.