Look before you leap here. Most Vendor systems, like Dell, HP, Sony, IBM, etc. use PROPRIETARY motherboard, specially made to fit their systems, by motherboard vendors. This means 'retail' motherboards might have PCI backplane slots in different locations or distance apart, plugs for ports like keyboard, mouse, USB, Network, audio, and telephone located differently too and you will not be able to mount the new motherboard. From the links you provide you can get a good look at those locations, although not too accurately, but probably close enough to determine if it might work. If you must, try and find a place that either guarantees it will fit or allow a return with no restocking fees.
I see someone has been able to overclock the CPU, see HERE.
I also see a Q8300 is a suitable replacement CPU in your present motherboard, see HERE but that may not be much better.
BTW, all you'll gain is possibly time. If the app is NOT built to use multi-cores (you already have 4) only the OS functions will use those, and adding cores might have little impact, other than the CPU speed itself and possibly some benefit of a larger CPU cache.