The editing of big documents, especially if there is a number of images in there, is telling. If the PC keeps almost "hanging" and you see the HD going beserk, then memory will be key. Having said that my 1GB RAM on this 3500+ CPU PC is doing just fine, and I can be editing word documents that have 30 pages worth of photos attached.
I quite like the SATA RAID I have on here. Don't worry too much about backups as if one drive dies the other mirror
should take care of it. On the other hand if there were a fire or flood
the 9800 is a quite respectable graphics card. It's all I have and does adequately for games. I'm surprised it's no good for your PVR use.
But yep there is no problem in you spending money, and building it yourself is straight forward, if you work out what to fit. Put simply I'd get the biggest CPU you can afford now, motherboard to suit, together with a nice case. You can compromise on the HD size and even have just enough memory for now. Those are easy enough to add more later. Don't forget the small things like a floppy drive - Windoze want to load most RAID drivers from floppy, before it will read the windoze CD for installation.