Partitioning my hdd

Hi, I want to put windows 7 on my iMac to play my windows games. I'm concerned about how much space to allocate for windows when partitioning my hard drive though. Do I reserve just enough for the win7 OS (I don't know what size this would be - but say for instance 25gb. Then thing is, when you install games obviously they need hard drive space, can you use an external hdd to do this so it's not using the macs internal hdd as I would have only allocated 25gb for the windows OS... I don't know if that makes sense or not. If anyone can shed any light, it would be much appreciated!

You're looking at reserving around 4-8gb of space for the windows OS (dependant on which Windows OS you are going to install)

- Simon

Well how much is available on your hard drive? Suggest halving what is available and windows will not function on an external drive. For instance on a 250GB SSD, after Lion and applications installed, 203GB remain and 100GB used for Windows 7 Ultimate partition using Boot Camp 4.0.

I was just going to get windows 7 home premium and I have a 500gb hard drive. So perhaps 250gb would be the best amount.
One other concern however, I love taking photos and have a large iphoto library. When I import photos I don't actually know where the location of the library is stored. Is it at all possible relocate the iphoto library to an external drive and change it somewhat so when I import photos in future they save to the external drive?... If that makes sense at all!