I always build my own.
2008 may not be a good year to buy (or build) a PC at all.
There’s a lot of significant long-term hardware changes happening at once right now - DDR3; 64-bit compatibility; Intel Core 3 Octo; depreciation of PCI, PS2, and LPT, etc. But much of the new technology will remain expensive throughout 2008.
So do I.
However, as of late, I find that using most older technology works just as well as the shiny new stuff.
I just bought a Pentium 4 clone desktop with HyperThreading for $75 locally. I put in some Corsair XMS DDR and a hard drive; the end result is a very stable, zippy system that flies under Windows XP.
Another system we have is a Pentium III. Not a Socket 370 processor but a Slot 1. Maxed out with PC100 SDRAM DIMMs, a GeForce4 video card, and it runs XP Professional just fine. We also run a Dell PowerEdge with dual Pentium Pros and ECC EDO DIMMs as a file server (got it for $25, including a loaded SCSI RAID).
Talk about ancient technology!
Pentium IIIs and Pentium 4s are dirt cheap now; add Ubuntu and OpenOffice, and you have an inexpensive and very stable computer for most tasks.
We mostly run Windows and Office, but the interface changes in Office 2007 and the death of Office 2003 are giving us good reason to consider moving ASAP.