If you can - wait. AMD is launching new CPUs and a new unified Athlon 64 socket on the 29th of March. That makes mid-April a great time to put together a new machine.
If you can't wait that long, there are price drops on current CPUs scheduled for around Valentine's day.
The current situation is not that great, especially from an upgrade standpoint. Both the Athlon XP and Pentium 4 boards currently available have almost no upgrade potential. The 3200+ will be the fastest Athlon XP available, at least that will fit on current Socket A boards. On the P4 front, the 3.4 GHz Northwood or Prescott is the end of the road for Socket 478. (Note that the older Northwood is a slightly faster overall CPU than the new Prescott, and much cooler running.) As for the current Athlon 64s, using socket 754, there won't be but one or two CPUs faster than the 3400+ released for socket 754 after AMD unifies the Athlon 64 and Athlon 64 FX to the new Socket 939. This means no current motherboards have but a one or two month upgrade path.
But wait, it gets worse... Intel's next "socket", LGA775, for P4s 3.6 GHz and up, won't be available for a few months. To make matters worse, there won't be an AGP slot, but rather PCI Express. PCI Express is the replacement for AGP and the current PCI slots. Unfortunately, this means that new graphics cards will likely be quickly transistioning to PCI Express, which makes video card upgrades on boards with AGP slots unlikely in a just a year or two. If that wasn't bad enough, DDR 2 memory will also start showing up sometime this year. Can it get worse, possibly yes, as Intel may also move to the BTX form factor for cases and motherboards as soon as this summer.
So, if you don't wait to build a system anything you build today will have no upgrade path for CPUs, video cards, memory, and maybe even the case itself in just a year or two. Basically, my advice is to wait and stick with what you have for a few months more.
