GTi_iTG here, changed my username
Honestly she would be more than happy with a 500-600 tower from Dell.
You dont need a Q6600 either, yes it will help but you dont need all that power just for photoshop, you cant count web surfing/downloading really, most newish pc's can do that with their eyes closed.
A higher end core 2 duo like the E6750- or 6850 would be fine with 2GBs of ram, again more would help but really it will be fast as anything with what i just suggested.

Another reason i suggest somewhere like Dell is because what happens if you put it together and something doesn't work? do you know how to make it work? if it breaks can you fix it again? if you build it yourself you have to think of these things aswell.

If the answer to all above is yes then by all means build a Q6600 pc and save some cash.