Be aware that motherboards are multi-layered and therefore not normally repairable with a conventional soldering iron.
I use a 12 watt with a selection of very fine tips for working on PCB's. But if you're going to be working on computer PCB's a lot it would be well worth investing in a temperature controlled soldering iron.
I am constantly fixing things like police scanners and kid's toys that have little circuit boards in them. My big Weller Soldering Iron gets too hot so I am thinking about getting a 30w pencil type soldering iron for small repairs. I am looking at this one >http://www.radioshack.com/product.as...5Fid=64%2D2067
I'm not sure if this'll work, but I've heard that a rear-window defogger-repair kit can be used to redraw cut traces on the mobo. Makes sense, since it's a conductor, but I have no idea whether it'll last or break or heat up or cause crashes or whatever. You might want to check that option out, though.
AMD Athlon 2500+
Abit NF7-S mobo
WD 320GB HDD
1.5GB DDR SDRAM
Samsung DVD-RW drive.
Dark Blue X-Dreamer II case.