Before buying anything new I suggest searching for used Mac Pros.

The 2006 Mac Pro with an x1900 or the 2008 Mac Pro with an 8800 are my top choices for Catalyst machines. In fact, all of my Mac Pro inventory is comprised of these machines. They are solid and work just fine with LFG4 or Phoenix cards.

Even the single processor versions of these two machines work fine.

There is no need to buy anything newer or faster in the Mac Pro line as you will not see any noticeable improvements. Also, I tend to notice that in the majority of my cross rentals that most rental houses use primarily one or both of these two generations more prevalently.

In the US I constantly find people with these two models posting on Craigslist for reasonable amounts and many are in the cities I happen to be in.