Take a look at these comparison for different computers using 3Ds Max rendering:http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=3641&p=5
The test scores for the i7-920 system was 15.3, for the i7-860 system was 15.0, and for the i5-750 system was 13.4 (higher is better). The i920 system will be about $100 more than the i860 system, and about $180 more than the i750 system. Only you can decide if the performance gains are worth the added cost.
By the way, the Anandtech article lists the exact hardware configuration for each computer, so you should be able to calculate the exact cost for each system using Newegg.com or some other online store.