|In my opinion, you should look to create at least 5 partitions.|
First partition for XP say 30gb.
Second partition for XP Games, programs and data storage say 150gb
Third partition for Win7 say 30gb
Fourth partition for Win7 games,programs & data storage say 150gb
That would leave about 280gb for a fifth partition for shared data such as music,video files,pictures etc.
Then you would want a separate physical drive for backups.
The reason I would keep the operating system partitions separate & small is for ease of maintenance. The system drives will probably need to be defragged more often than the data partitions and the smaller they are the faster the task will be done.
Goin' Fishin' (Some day)