Gecko, Conceptual, not trying to be defensive or to put anyone off from learning CFD. Far from it. I just wanted to get more information about what the computer was going to be used for. Valves, pumps, electronics, or full-scale F1 simulations.
As far as machine specs, Gecko is correct in that the size of the simulation model that can be handled is dependent upon RAM, while simulation speed is dependent upon CPU speed.
I would avoid the current quad-core processors for computationally intensive simulation programs like CFD. In our testing, we have found that the front side bus bandwidth will be exceeded before the processors are fully cooking. A four core unit with two cores solving may produce a ~70% improvement in speed. Enabling a third core may result in 75%. A fourth, 76%. Until the FSB technology is dramatically improved (or eliminated altogether) these quad core processors can't be fully utilized.
I believe both Intel and AMD are working on new processor/memory pipeline technologies (point to point interconnect) that will replace the current, out dated FSB technology.
I'd recommend buying a faster dual core processor over a slower quad core unit. Or if it's in your budget, two dual core units will provide better performance than one quad core unit.
8GB of RAM will allow you to run a simulation model with 10-15 million elements or 3-5 million nodes (depending on the type of mesh you are working with).
I wouldn'nt be overwhelmed about learning CFD but I would be if it involved learning how to use an independent mesher, what all of the solvers mean and their capabilities, pros, and cons, how to interpret results, etc.
Telling people that they should back away from CFD is not what I intended when I said that traditional CFD should be left to the analysts. The company I work for was founded on the belief that there is a better way for mechanical design engineers to use CFD in their design process without being CFD experts. There is no reason why CFD has to be difficult. Sure, there's a lot of stuff going on behind the scenes and a basic fluids course only scratches the surface of the physics involved, but frankly, MEs don't care about partial differential equations and "this solver works better for this application" sort of stuff. They just want to know if their design changes are improving performance without testing them manually.
If you are just looking to run random projects here and there for your interests, I think your only really feasible option is to download something like OpenFoam since the costs to get a decent all in one CFD package would be considerably more than your machine alone. There are better ways to learn CFD, but unfortunately, they aren't free.