I think the general consensus here is the i5-750, but it really does depend on what exactly you want to do with the system.
So, Mr OP, let us know what you want to use your shiny new rig for and the CPU choice will become clear.
In a nutshell though, it seems to go like this:
Purely gaming with a single but powerful GPU: Phenom II X4 (might as well go 955 BE) or Core i5 750
Mid-level number crunching (i.e. fairly complex spreadsheets) and mid-level design/media (graphics, CAD, video): Core i5 750
High-level gaming with multiple GPUs, intense number crunching and top-level design/media: i7 930
All those CPUs can be massively overclocked as well, pushing performance up to the "tier" above.
If budget is very tight, or you're not doing anything major with the system then yes look at dual-cores: the Core i3s are overclocking monsters apparently.
Good questions back to me. I have a mechanical engineering company that uses Autodesk's Autocad program. Which has 3 D capabilities and planned usage for 3 D. There are different levels of complexity when you get to graphics programs, those that remain staic and those that are going to be used in a dynamic or video application,which you created. Doing visual fly through spaces is fairly common, althogh I have no idea how to do it, I am just the boss. I personall do get involved in the calculations side. There are several energy simulation programs that will perform complex high level mathmatical computations 8,760 times for every surface you define in the program. There are 8,760 hours per year and the weather parameters, temperature, humidity, sun intensity,angle of incidence, etc. This one program developed by Department of Energy in late 70s early 80s used to take me 4 to 8 hours to run for a large building. With advances today in computational speed and processors that same building could be done in 4 to 8 minutes. That still is quite a drain on a system. Imagine all CPU power running balls out for 5 minutes to get one answer. They have made these programs with the end user in mind and can be run on relatively cheap graphic and processor speeds. On a scale of 1 to 7 the CPU should be a 5 or better and the graphics should handle 3 D and should be about 5 or better. None of my engineers use laptops, they get burned up way too fast and the heat removal is a problem. Thus desk tops is the only pragmatic way of handling crunch time. I can easily use a 2 year old Laptop for showing the end product soemone is getting but it would not handle the journey in how you got there. My firm is small right now and I just blew up a good Pentium 4, 2.6 MHz, 2 MB Ram, 3 D Graphics card (forgot name) 516 Mhz, desk top computer. My programs prefer Windows XP Pro.
I had this $1,000 figure in my head I wanted to try and stay within for the main box. I plan on using two 21.5" Dell flat panel monitors for a combined priced budgted of $400. I am not a computer expert and do not have to have to hotest thing on the market. I do believe in a few principals. unless there is a compelling reason buy the 2nd best out there, buy a good monitor you will have this for 2 to 3 life cycles of the CPU. I have one 17" wide screen format HD Sony monitor that produces brilliant reproductions. Bought it 4 years ago for $1,000. Keeping it.