Partitioning advice for SL, ML, W7 and my stuff

I currently am running Snow Leopard, Windows 7, and Ubuntu. My 500 GB hard drive is partitioned approximately to 450, 25 and 25 respectively. I'm planning on ditching Linux and repartitioning for SL, ML, W7 and a data partition to share between SL and ML. I'm looking for opinions on what would be good partition sizes for each OS, considering my data will be separate. For example, 30 GB each with the rest for data? Is that enough? What should I be considering? I have the feeling I'm not thinking of something vitally important.