If you are going to invest in a Xeon system with multiple processors, I'd advise you to wait if you can. The current E56xx processors are quite old, and due for a refresh in November. Most of them came out around March 2010.

If you are eyeing Xeons in the E3-12xx range, they are mostly just rebranded Core i5/i7 chips, and work in a single processor setup only. Better to just pick a Core i5/i7 based system at that point.

And I will put in a serious recommendation for looking at a Mac Pro if you wait for the new Xeons. When I bought mine in 2008, I priced the identical config against Dell Precisions, HP's workstations, and one other company I can't remember now. Even with the cost of a Windows license added to the Mac Pro, it came in cheaper then Dell and HP, and it's been a very solid machine. I think peter here did the same at some point.

Getting XP to run on a new machine, Mac or not might be tricky though, since it is well past the supported timeframe and newer motherboards may not even have drivers for XP. Windows 7 does have an XP mode with Professional and up. I don't know how well it would work with the apps that are XP only, but may be a solution. You would need Win 7 Pro anyhow, since home only supports one processor.