Upgrading a laptop from Core Duo to Core i series is impractical. Sony don't sell their motherboards separately, and laptop motherboards are custom jobs. Not like desktops, where most motherboards fit most cases.
Are you sure their aren't used or refurbished motherboards from the Vaio FW series laptops that are available? My quandary is that the Vaio FW 490J that I have already has a Blue-ray burner, a 7200rpm hard drive, a FULL HD 1280x1080 screen and an ATI Mobility Radeon™ HD4650 graphics card with 1GB vRAM. So it is my perfect laptop except the motherboard can't be upgraded to an i-series Intel processor. I use almost all of the Adobe Creative Suite programs and usually have several open at once and the Core Duo 2.8ghz struggles to keep up sometimes.