I'm not sure how to answer your question regarding whether or not you would lose your preinstalled software. I will do my best to answer.
1. You won't lose it if you just upgrade, but upgrading can and might "break them".
2. If you upgrade by doing a clean install, yes you will lose them unless you have the installers backed up or if you can redownload them from the manufacturer.
3. Most of your software will run on Vista, some won't work at all.
Vista Ultimate does have Media Center. From what I understand it is a vast improvement over XP's.
One other suggestion:
My buddy has a laptop with XP on it. He says the battery lasts about 3 hours.
He upgraded it to Vista Ultimate. He says the battery only lasts about 30 minutes now.
Warning: Vista may drain laptop batteries