cool...a thread i can speak directly to
The only way to tell if you can flash your card to a 9800xt is if you remove the cooling and look at your core. If it says "R350," DO NOT FLASH IT TO XT!!!!! You will artifact all over the place and according to rage3d.com forums, you will lose the ability to run AA. However, if you remove your cooling and see "R360" (the XT core), then go for the flash. Supposedly, newer 9800pro's being sold as OEM's are being shipped with R360 cores.
As far as modding your card, there are 2 schools of thought. The first school says, why would you even bother when the card runs past XT speeds as it is? The other school (which I belong to

) says, "heck, mod the crap out of it to squeeze every FPS that you can out of it!"
First, improve the cooling on that thing. I decided to go with the Arctic VGA silencer, partly because I was looking for a simple, bolt-on solution to cool this thing. My core overclock went from being stable at 405 mhz to being stable at 420mhz. Not bad. My Ram wouldn't get past 360mhz, but we'll get to that in a minute.
After reading
this article, I decided to volt-mod my card. I solder onto circuit boards about 3 days a week because of work, so I found these volt mods to be easy. If you have any type of background with soldering and using a multi-meter, I recommend this mod whole-heartedly (just be careful when doing it!!!) My core overclock has since gone to 440mhz, and my Ram overclock has gone to 365mhz (not great, but I don't have any Ramsinks, and my ram chips are hot as crap to the touch, so heat is limiting me on my Ram now).
So, be sure to check to see if you have an R360 core before flashing to XT. Also, check over at rage3d.com forums, or ask here, or PM me if you have any more questions.