1. case
Antec Sonata II - $120
The SLK3000 cools better, but the Sonata has more style with the glossy black paint job, plus it comes with a pretty nice PSU.
2. power supply
450W SmartPower II comes with the Sonata - Free
Not the beefiest PSU in the world, but it is more than enough to run what she needs.
3. cpu
Athlon 64 X2 4200+ - $559.00
Dual core all the way. You could probably save money by getting a nice single core processor, but the dual core chip is going to feel a lot smoother to her during normal day to day usage once (virus and adware scanners running in the background while she is playing MP3s as she does her normal computer work), so she'll really feel like she's getting her money's worth.
4. board
Chaintech VNF4 vanilla - $76
If she's not overclocking, she probably doesn't need anything too fancy. This board has the NF4 chipset and PCIe graphics and plenty of SATA ports. It'll get the job done and it doesn't have an annoying chipset fan like most of the other NF4 boards.
5. memory
Corsair Value Select 1GB kit (2x512MB) - $79.99
The A64s don't need anything fancy and 1GB of RAM should be enough for normal use right now. If you need 2 gigs, you can always put in another pair later. You're not going to be running 1T with the cheap stuff anyway.
6. hard drives
Seagate 7200.8 series SATA. The 250GB will run you around $130 or so, and it is a good place to start capacity wise. Get a second drive and mirror them if she has any data she seriously needs to keep.
7. optical drives
NEC 3540 DVD+RW - $50 and dropping.
Does everything you need an optical drive to do. You can also get a version of the slightly slower 3520 that comes with Roxio Easy Media Creator 7 for around the same price.
8. heatsink/fan
Zalman CPNS7700AlCu - $34 at Amazon.com.
She'll likely want a quiet system and this HSF fits the bill. If she doesn't mind a bit more noise, you can stick with the stock HSF that comes with the retail X2s.
10. lcd
Dell 2005FPW - around $500 (depending on the coupons available)
Wider is better. Also, I'd personally stay away from the LG90+ and other 8ms response time LCDs, since are 6-bit dithered panels and don't reproduce colors as well as the slower 8-bit panels like the Dell Ultrasharps. Only hardcore gamers that can't stand to see ANY ghosting are they only people who need the 6-bit panels' speed. For general useage an 8-bit is usually much nicer. The similarly priced 2001FP has a bit more screen real estate if she wants maximum workspace.
11. video card
GIGABYTE GV-RX80256D Radeon X800 - $240
A good card, but not overkill like the 7800GTX is. It'll let her run just about any game pretty decently on the Dell. It is passively cooled, so it won't be noisy in her quiet case. The Gigabyte X800XL and 6800GT cards are an option if she needs more gaming power. If she is just going to be using it for office apps and the like, you could probably get by with a passive X300 or 6200TC and save some cash for other nice upgrades like an Audigy for better quality sound.
The running total for all this comes to around $1800 or so once you factor in tax, shipping and whatnot. With the extra $200, I'd recommend also getting Windows XP MCE 2005 for the OS. You need and OS anyway, and MCE has most of the goodies that XP Pro does plus the Media Center features. If she wants to watch TV on her computer, all she'll need is a decent tuner card (like a WinTV PVR 150 or a Sapphire Theatrix 550 for $70-$80) and MCE will take care of the rest.