I'm wanting to build a system to run preferably 2 versions of Linux. I don't really need anything special right now, probably the main things I would like in the system currently would be a nominally decent overall spec with enough storage to let me try out various things/apps in Linux. I have never built a system before though. I would like the system to to be easily upgradeable so that when I become more familiar with Linux I can migrate over to it and upgrade it to make it my primary PC over my current XP sys. Does anyone have any suggestions for a basic build, preferably no more than about $500? I've looked and I feel a little overwhelmed, especially since I am not overly sure what is necessary for a good Linux box. Any suggestions would be apppriciated.

Decide which linux distros you may want to try and than check out their Hardware Compatibility List. One key ingredient for a good linux system is getting hardware that's compatible with it. Not those Winstuff/hardware (winmodems, winprinters, etc).

Hopefully this will give you a general idea on building your system. Also you may have to spend a little more for good hardware.