If I were you I'd look to get a motherboard that can handle i5/i7. If I recall correctly, i5 is the "middle" and i7 is the "extrememe" level of the cores. There are some 1156motherboards out there that look pretty decent, but if there's wiggle room go for the 1336 motherboards - from what I understand they will have the best chance with the i9. Intel changes often so who knows really but that'd be your best bet from what I've heard.
You could get your daughter a PCI-Express card for $100 or less and it'd likely do WoW without too much grief. That'd be a good start I guess for her.
If so I guess I'd go for the upgrade you and give her recycled parts. I just saw a SATA 6 1TB Western Digital Black drive hit Newegg today for $90 (Coupon code: EMCYSYR59).
So my vote is: Get a 1336 motherboard with an i7 processor, some DDR3 RAM, new HD, new PSU and recycle your video card into the new rig. Then get an inexpensive dedicated graphics card for your daughter to put in the old setup.
I specced out an entry level i5 with 1336 motherboard as just a basic "office" machine for $600ish.