FAT32 will not cause any problems with XP - it's one of its native filesystems! It's not efficient in terms of storage, especially on larger partitions, as small files can take up quite a bit (relatively speaking) of extra space, but there is nothing overly slow or dodgy about it. There is another limitation to consider - FAT32 cannot handle huge files, as in over 4GB, so if you are planning to store whole DVDs on your large drive, you will need to look at which system needs write access for that sort of thing.
If you would like to try Reiser, you will need an add-on for XP, like this one http://p-nand-q.com/download/rfstool.html
. I am not brave enough to try other filesystems under XP, so I can't make a recommendation for that software. FAT32 would be the safest and easiest way to go - there is little chance of data corruption from either system.
I don't use thunderbird, but bruno seems to have answered that for you.
As for Opera - this could be a little tricky, but I think it is doable. You need to keep separate copies of your opera6.ini file because they store whole paths, which aren't compatible between the systems, but you could store the other files from your profile directory on your FAT32 partition, and update the paths in both system's INI files to look there instead. I do this to share my bookmarks, but I haven't tested more than this.