I just did the exact same thing.....currently have an XP2100 overclocked with the Asus A7N8X Deluxe mobo and 1Gig of ram.
I started researching and put together a system at a reasonable price that can handle the Vista upgrade when it comes. After rebate it came out under $450 with EVERYTHING NEW!
Abit IP35-E motherboard - Rebate at Newegg
Intel Dual Core E2160 that I am going to overclock a bit
4-1GB Chips of the HP Ram on rebate for $10
Samsung DVD Burner with Light Scribe for the heck of it
Antec 380W power supply. I would also consider the Antec Earthwatts 430. - Both are on rebate at newegg
Rosewill case that is decent for $60 shipped
Got a CPU cooler for $1 after a $20 rebate
Got a 500GB Sata drive from Frys
256MB video card that is somewhat compatible with Vista for $30 after rebate at zipzoomfly.
I am waiting for the processor to show and then its build time!