Yes, and for the Peg you must be sure to get a 9800 that universally keyed (so that it will work in the 3.3v AGP slot the Peg uses).
The card edge of a universally keyed R300 is dived up into three sections with two slots separating the middle section.
A 4/8X R300 has only two sections with one seperation, and usually these will not work at the voltage provided by the Peg.
This can get a little weird in that some later 9800s are based on older board designs, but use the last revision of the gpu (R360 I think, if I'm wrong Andreas will correct me).
So, while it would seem to be impossible to use a Radeon 9800XT in a Peg (as they are 4/8X cards), some of the cards that are universally keyed and are labeled as 9800Pro cards actually carry a 9800XT gpu (and will probably overclock).
It probably wouldn't be too hard to modify on Peg for a lower AGP slot voltage, but since the firmware won't recognize later cards, there wouldn't be any point in it.