That makes sense. No, you would not be able to install any version of OSX on that machine. hackintosh OSX only works with very specific hardware configurations. And is not very stable. If you are not very familiar with PCs, I wouldn't even try to make it a hackintosh. It requires a lot of BIOS manipulation and computer knowledge. However I love windows 7, I don't think you'd have too many problems making the switch.
Yes, xeon chips are great but you only get about 10-20% increase in speed for a 50%+ markup in price, so it's not really worth it. Like you said I much prefer the i7 series for cost vs. performance.
I would spend the difference on a better graphics card than the GT 660. Is there the option for at least a GTX 680? the 6xx series cards are not very efficient at CUDA and compute performance.
Last question, is there any other company who builds computers in your country, like a smaller online business that could ship locally? Or even an individual who builds computers on craigslist? You could get a much better deal on a system and be able to customize it for your needs, instead of spending more to customize a pre-built Dell.
good luck, if you have any questions about parts feel free to ask.