Just get an Athlon 64. It will run everything the XP will and faster.
When the 64 bit OS you want appear, switch over to it. In the mean time
stick with 32 bit windows.
With Windows XP 64 bit then you can concurrently run 32 and 64 bit apps.
This is the advantage that the Athlon 64 has over EG Itanium.
Many 64 bit systems are being sold and the prices are very competitive - it
is the upgrade path now.
Windows XP 64 bit is plodding along quite well from what I see (I don't have
it as I don't need an upgrade). Driver support is improving slowly but
surely. The product is still in beta, but many are using it. Personally I
would not use it for production without a thorough evaluation and research.
- Tim