You are attempting to get something for free. A 9800 pro for the price of a 9800SE. I would put up with any razzing you get for buying that card. Because if the soft-mod works, you'll be a very happy boy. If it doesn't, You are stuck with a card that is outperformed by a 9600XT.
Remember when the 9500 non pro with L-shaped memory could be modded to a 9700? They had a pretty high success rate. It was all over the web in almost every forum.
The 9800SE however is a different story. Although with extreme luck it could work, but I hear more warnings to not do it than happy stories about how it does work, if any.
Only attempt this mod if you are 100% sure that you can undo it if something goes wrong. Otherwise, you could be stuck with a paperweight.
I would like to hear from soneone who has done this successfully. I have even heard stories about successful mods actually being 5% slower than the 9800SE was in the first place.
Keys