I would say to wait for the ATIs. It'll be worth the wait, and by then AMD may also have their 4x4 Quad core rig out too, which should be awesome. It's dual proc, but it's supposed to be awesome.
RB, I think thought that the upper limit on 32bit XP was like 8gb, where 64 was like 16 or something.
From
http://support.microsoft.com/kb/555223:
Here's a list of how much RAM the various Windows versions and editions support (as of Nov 2004):
Windows NT 4.0: 4 GB
Windows 2000 Professional: 4 GB
Windows 2000 Standard Server: 4 GB
Windows 2000 Advanced Server: 8GB
Windows 2000 Datacenter Server: 32GB
Windows XP Professional: 4 GB
Windows Server 2003 Web Edition: 2 GB
Windows Server 2003 Standard Edition: 4 GB
Windows Server 2003 Enterprise Edition: 32 GB
Windows Server 2003 Datacenter Edition: 64 GB
I always say that if someone is going big, they should go all the way. 64gb ram with Windows Server 2003 Datacenter Edition. Just load up HL2 or any other game or games for that matter into a RAM drive, and watch the framerates fly...