You can have the OS on one drive and data on another drive, no problem. There are tools that will move the special folders (e.g. My Documents, My Pictures, etc.) to wherever you want them to be. For example, Tweak UI can do this. Many people prefer to have their data well away from the OS.
Partition Magic is long discontinued. The free Easeus is almost identical. Again, there are many tools that will let you create and adjust partitions. The operating system can also (usually) create partitions, but often not with the flexibility or ease of a third-party tool.
Re partition size, you can get XP and Win 7 into 5GB to 10GB, I'd guess, but you need to consider the size of the swap file unless you've got the gumption to move it somewhere else. Also, even if you religiously install applications to a different partition, a lot of cruft still gets added to the system folders over time. Personally, I'd probably use 30GB for the boot partition since drive space is really cheap. In reality, I've got a 1TB boot drive and two 2TB data drives on one of my machines, mainly because I couldn't be bothered to buy anything smaller.