Just to clarify: you are looking for a new system under $500 for yourself, and will be handing down all the parts listed to your daughter with the exception of the hard drive (you want a new one for her), correct?
There is nothing wrong with this (thats what I did on Passion), I'm just clarifying. Also, any preferences on form factor? There are some good combo deals on MicroATX on Newegg right now, one of which I list below.
Total: $322
Out of all this, I would say the combo deal is the one thing you can think about upgrading. I don't know what your card situation is like, so moving up to a full atx with a quad core could be an option. I went with AMD because currently they are the best bang for the buck, especially with the combo deals.
For the RAM, idk if I'm out of the loop but it seems $75 is high for a 2x2 set, so someone may find something cheaper. The RAM can also be changed out, but that is simply if you don't want to take the time to tweak it for top performance. Out of the box it will still be fine.
The 320GB Caviar Blue is the best bang for your buck on the HDD, unless you really want a 16MB cache.
That PSU is my favorite out of them all. If you aren't running two cards, the 550W continuous will be plenty for you, and it is modular and currently at a great price.
As you said, timing is everything, and that combo deal and the discounts could be gone tomorrow. Let me know if you have any other requirements and I will gladly revise the list. $322 is a great price for that 3/4s of a system, and if I had it spare I'd probably snatch up that set right now. As you probably know, $322 will let you do a lot more this Christmas than the full $500, but I'm sure you will consider that for yourself as many of us will have to this year. This was also a first look through on Newegg, mainly with Low Price and Best Ratings searches, so I may come back and revise it on my own later.