Gaming PC Upgrade/Build

I've been given an HP Pavillion desktop which I want to upgrade and use to replace my aging gaming PC (it's almost 7 years old and I want to be able to do Ony 25 in WoW without whelps making me DC).

The Pavillion has a Pentium 4 515J CPU, some custom ASUS motherboard, Radeon X300 graphics and a gig of DDR2 RAM.
I want to do the upgrade in stages, starting with moving another gig of RAM in from my old pc as well as a bigger SATA hard drive that I already have.
Next is what I would like advice on.
I want to do the upgrade in stages, first switching out the motherboard, then the graphics, then the cpu and maybe a new powersupply if 400W isnt enough after the upgrades.
Basically I want some advice on what motherboard to get. I'm thinking of going with ASUS because of the excellent overclocking ability. I need to find one that will support both the P4 515J (prescott) as well as a newer Core 2 Duo or Quad.
Any advice anyone can give would be very helpful,

Thanks - Bob