Designing, making and installing this daughterboard will be a huge amount of work.

You'll probably want to use a XScale as a CPU, you'll need RAM on the daughterboard as well since the existing RAM isn't going to be fast enough, the USB 2.0 controller will require another chip which can attach to the PCI bus and as you're doing all the other upgrades then you might as well put in a 10/100 ethernet PHY. Oh and don't forget that since you're upgrading the display as well then you'll need to put a connector on the daughterboard for that.

All that's really left on the main board that you'll still use is the Phillips DSP, Crystal ADC and HDD connector. You might as well start from scratch if you're going to be doing all this since the daughterboard will contain nearly everything by itself.