I would not invest in anything less then a Mac Pro 3,1 and even then I'd be hesitant, however i realize that many people are constrained by budget and want to pull the trigger on something, I'd advise they wait until they have just a little more money.
The first two model Mac Pro's 1,1 and 2,1 were hampered by slow bus speeds, 1.33GHz, slow ram and limited CPU upgrades. There is no getting around the slow bus speeds and ram, both of which are woefully slow by todays comparison. Yes, you can upgrade to dual quad core CPU's, but any newer graphics cards you use will run at PCIe 1.0 speeds no matter what, so that will be a bottle neck. You certainly can put in an SSD for some added speeds, but thats really all you can do. Now, all of this might produce a machine that is fast enough for you, and that is fine, but if you want keep this machine for any length of time, 1-3 yrs, you are quite limited.
In my opinion, it would be better to buy at least a stock 4,1 system, although I realize they are hard to find now and costly by comparison, your options in terms of upgrades are vast. Even a 3,1 is limited by CPU upgradeability, slow ram, and only two PCIe 16x slots.
I've owned a 1,1 and 3,1 so I know these machines, what they are capable of, how they perform etc. I now have a stock quad core 5,1 system and I have lots of upgrade options the least of which, upgrading to a hex core cpu, will increase my geek bench score from around 10k to 15k and make the machine a lot snappier. By contrast, similar cpu upgrades on older machines might produce virtually the same geek bench increase but in reality the machine is not much faster after the upgrade do to technology limitations like bus speed. So your money does not go as far in the older machines as it does in the newer ones.
In the end though, as I said, if you wind up with an older machine which, after some upgrades, is fast enough for you, that is your choice and there is nothing wrong with it. This is really just to illustrate the limitations of the older machines to help people make a more informed decision.