Here's what I do-
Use a basic windows XP on a Pentium 3.3Ghz machine with 2Gb Ram for most stuff around the office, MS word, Excel, Photoshop, PageMaker7, Internet and e-mail. NO Video Editing on this machine. It is what I call my office. It has the accounting stuff on it with Quicken and some other apps as well.
Then I have 3 other decktop towers that are buit with video editing in mind. These have only editing aps with multiple USB drives, as well as SATA Raid internals and high speed graphics. One is a dual core, the others are fast Pentiums. Two have 1394 VCR connections. I also have the new laptop which does all office and video editing.
The point is, I separate the video editng from the office functions. You might consider doing the same. Keep a lower end office machine and use that X64 with the intended editing software.
I also have a computer in the network that runs WIN95 OSR2 for applications that work best on that OS. Then I have an older Pentium that runs DOS 5.0 and QEMM for 3D Studio 4.0 but I rarely fire that up as I refuse to do 3D animation unless someone really wants to pay my rates. I think some day your X64 will do it all for you but you stated the reason you went with that OS, so use it for that! Then get a normal machine to do all your other tasks. BTW- The new laptop is just blazing fast on video editing and at dual core 2 2.33Ghz. equal in speed in video rendering to my 3.8 Ghz Pentium water cooled machine. I am very very happy with this new Dell laptop for everything I've thrown at it. Even the Express EVDO internal card connects to the internet at speeds of 1.3 Mbps average which is about 4 times faster than my old PCMCIA EVDO card.