AutoCad likes big, powerful processors, but not hyperthreading (actually hurts performance). It's also one of the use cases where Bulldozer/Piledriver fall flat on their face due to the shared FP scheduler, so stay away from those. So we're looking at Intel chips with lots of cores and high clock speeds, but not their top of the line chips with hyperthreading.
Really, it's about how much you're willing to pay and if overclocking is an option.
For around $220, you can get a 2500k. If you overclock, they'll hit 4.5 GHz easily with a cheap cooler. $210 for the 2500 if you aren't going to overclock.
Go up to $230 ($215 w/o overclocking) and you can get the 3570k. About 8% faster than the 2500k/2500.
Higher than that and you're looking at either a Xeon or Sandy Bridge-E chip. The value prospect here is murky at best.
Switching to a server chip, which requires a special motherboard, the Xeon E5-2620 is a six core chip for $420. It does have a very low clock speed, and would probably be outperformed by an overclocked 3570k for about half the cost.
At $570, the 3930K gets you six cores that can be overclocked even better than the 3570k. Add in the cost of a cooler, however, and this is a $600+ chip. It costs more than twice as much as a 3570k, but will not give you twice the performance. Up to you to decide if another $300 is worth a 50% or so boost in CPU performance.