Honestly, most major distributions of linux have builtin partitioning, boot manager tools to make it as painless as installing an application. Some things to think about is :
1) KDE vs. Gnome vs. Other windows managers. KDE, then Gnome, then other Window Managers take different levels of RAM to run. Any machine with about 512 it shouldn't matter which one, but some distros (like Kubuntu / Ubuntu / Edubuntu) have one type of Window manager they use. Some have all of them.
2) If you want to be able to interchange (like me) files in-between, you will want to set up a FAT32 Partition, but if you just want it for internet / programming / whatever, then you probably don't need one.
3) Package Management to add / remove apps. It does make a difference, because there are some distros which have more development in the way of apps.
4) If you want to run games, you need to research what it will take to make some components work. ( I had a friend build a powerful machine but the ATI card was a pain in Linux. ) That goes for all the other stuff you are thinking of doing.
5) Research local LAN file sharing, because some things ( like Kubuntu / Ubuntu have steps beyond just clicking a button to get it to work.) If you don't have a need for it, then this can be skipped.
I would say, IMHO, Kubuntu / Ubuntu by going to www.ubuntu.com
and looking around. For lower powered machines, DSL @ damnsmallinux.org works really well. Although its a little less user friendly for install, its one of the best (IMHO) out there.
Good luck, and if you have any questions, well, just post back here.