It's kind of a wash. I would second what others have said here. But, to answer your question a little more directly - a 9800 XT is, basically, an OC'ed 9800 Pro 256 MB. So yes, if you buy a 9800 Pro 256, and flash it with the XT bios you will have an XT. Or better yet just OC it to its max stable OC.
You say "massively OC" which is certainly possible, WITH good cooling. Many 9800p's come with cheap cooling, that's why they're so cheap. I have a Zalman on mine (r360) and run 430/740. Higher is possible. With any luck you will get an r360 core, as most pros ship with the r360 (the XT core).
Be very careful you get a "real" 9800pro. I.e., 8 pipes, 256-bit memory controller. Some products are labeled pro but they're really SE.
EDIT: My recommendation: save your clams for something better. Don't spend more than $130 for 128MB ver or $150 for 256MB ver. Perhaps a bit more than that but you will wish you got an X800 later 🙂