I'm not sure about lag really. I simply can't comment there. Render times would definitely be longer. What are you renderer are you using? If you're going for a used mac pro, I'd suggest looking for a good deal on something like a 2009 model.
The cheapest 2009 model would be fairly comparable to your i7 920 in raw cpu power. It would be new enough that upgrading the gpu will probably be an option with whatever comes out for the next mac pro generation. The ram is significantly cheaper than ram for the older ones in case you're upgrading ram. 2006-2008 used FB dimms. They were pretty expensive. A few people on here have mentioned finding used 2009 mac pros for a little over $1000, so they don't have to be terribly expensive.
I really don't care for the imacs (they run pretty, glossy display with led backlighting that isn't very good for color grading and the glossy effect tends to hide fine details if any light is hitting it), but "in cpu only" (and assuming temperatures stay low enough that they don't have to throttle down), the i7 imac is faster than the slowest quad mac pro. They should be coming out with a new mac pro by early next year. This may put a lot more 2009/2010 models on the used market, but in terms of single socket workstations, you pay a lot for what you get with Apple. The single socket mac pro is one of the things that even the real Apple fans say is priced pretty high. Edit: the first few versions were actually priced really competitively, it's just since 2009 that a single socket workstation goes for that much
