"Is this a good idea for sharing data"
this is what most people do
there is another option, create a fat32 but this creates file size limit of 4 gbs(fat32 limitation)
also you can install a ext3 driver to be able to use a ext3 partition on windows xp, but only ext3, ext4 doesn't have support yet afik
usual procedure is leave a decent space for linux, a decent space for xp and a huge space for that shared partition
based on those two partitions you mention it seems that oyu have a 500 gb hard disk, i would leave for xp 60 or 80 gbs, for linux 60 to 40 and the rest for that big partition to share files between both os