No. Real world difference between the 3GHz P4 you have now, and the 3.8GHz P4 you said was the max the board could take is rather small. More so if you consider then something even as lowly as the C2D 2180 or 2200 will be faster then that 3.8GHz P4. Toms did an article awhile ago showing even the dual core Atoms beating the P4s. Intel did everything they could to increase the P4s frequency, at the cost of the chip being able to do real work.
You don't need to spend $2k on a new computer. If you have a S775, you most likely have DDR2 ram. If you bought an AM2+ motherboard and Phenom 2 CPU you could have a MUCH faster system for probably less then $200. Just reuse all your old stuff.