You should go to http://wiki.osx86project.org/wiki/index.php/Main_Page and find a version of OSx you want to run. Then you should look over the compatibility list and piece together x86 hardware to build a working machine. Then I suggest you install OSx and Windows XP so you can load windows when OSx won't work for your applications...
OR be a nerd and build a very high end machine with quad core processor, a shit ton of memory, 2 identical high end video cards and monitors, and run parallels with windows xp on your right monitor and OSx on the left as normal. Then you won't have to dual boot.
You will probably spend less money going with my first suggestion then going to the Apple store. But whatever works for you is fine.