The case is fine.
The Thermaltake is okay, but since you'll be overclocking I'd recommend something better.
This Fortron will handle all of that hardware plus any overclocking you do.
The Thermalright XP-90 comes in both aluminum and copper forms. The copper one costs over $70, while the aluminum is only $30. Despite the fact that copper is a better conductor, it will only mean a few degrees.
Get
this or
this for your hard drive. The second one is over twice and large and will be significantly faster. I just listed the first because you might not need that much space. You can also consider the Western Digital 10000RPM Raptor. The drives I linked are both SATA II and are quite comparable to the raptor, despite its greater spindle rate. It will probably be a little faster for some things, but it's a lot smaller, so IMO it's better to go with a bigger, SATA II drive.
The CD will not work on your computer at all. It is for her machine only. Even with the code on the back of her machine, it might not work, and if it did it would cause her OS to stop functioning. You'll have to buy a new OS. Get either XP Home OEM or XP Pro OEM.
Tricks? Well, some items (mostly fans, cables, and other small items) give you a volume discount, meaning you get more. Some items (e.g. a DVD burner) can be purchased with other specific items (e.g. a 50-pack of DVD-Rs) for a reduced "combo" price. Some items also have mail-in rebates, which will be shown below the price. Subtract the MIR value and you'll get what it will actually cost you in the long run. Other than that, there's not a whole lot you can do. You'd think they'd give you reduced shipping when they pack multiple items together, they don't. The shipping rates are fixed beforehand, so it doesn't matter what the actual shipping cost is in the long run.