Here's an idea, BruceB... what about getting a small dual-drive NAS for your LAN, running in RAID-1 (mirroring) for your office files, and then getting a newer, faster, desktop, but keeping your older P4 PC on the LAN, but running remote-access software on it.
That way, you could access your office files from either machine (because they would be moved to the NAS), and you could do your web browsing on the modern machine (so no performance issues there with browsing), and still be able to remote-desktop into your older PC to run your software that's still installed on the old machine.