Case: Up to you, ~$50
CPU: AMD PII X4 955 BE, $160
MOBO: AM3 mobo from Asus or Gigabyte, $60-120
RAM: 4 (2x2GB) DDR3 1333 or 1600 ~$110
GFX: What's her monitor res?
PSU: 500W-700W from Corsair, OCZ, Seasonic, PC Power & Cooling, or Antec, ~$40-80
ODD: Cheapest DVD burner on Newegg, ~$23
You think you would really see the difference in the cpu u listed vs if i put that money saved into the gfx card?
She probably won't notice the difference plaing Sims 3, but I recommend at least getting the Athlon II X4 620. Not much more expensive.
Does she plan to upgrade her monitor before (if) she upgrades this system again? If not, look at a HD 5670.
Be careful with OCZ RAM; some models are great, the rest are fickle and don't work with many mobos. The set you linked is one of the latter. Take a look at the reviews.
Also, this Seagate is a much better HDD; it's faster, more efficient, quieter, and 180GB larger. And it only costs $5 more.
They already have a HDD.
How much future-resistance is desired here, i.e. will there be upgrades in future years? I'm sure a better monitor is in your friend's future, which suggests that even if it's overkill today, a HD5770 is probably worthwhile, or at least a HD5750. If Sims 3 and Diablo 3 are the most demanding games your friend will play, even a HD5670 should be enough, up to 1680x1050 (if it were me I'd still prefer a little stronger though). You don't list an OS among what you have already, so figure $105 for Windows 7 Home Premium.
Here's a quick take on a list:
Antec EarthWatts Green EA-380D Green 380W Continuous power ATX12V v2.3 / EPS12V 80 PLUS BRONZE Certified Active PFC Power ... - Retail
Model #:EA-380D Green
$59.99 -$20.00 Instant $39.99
SAPPHIRE 100284L Radeon HD 5750 1GB 128-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFireX Support Video Card - Retail
$144.99 -$15.00 Instant $129.99
AMD Athlon II X3 435 Rana 2.9GHz Socket AM3 95W Triple-Core Processor Model ADX435WFGIBOX - Retail
Microsoft Windows 7 Home Premium 32-bit 1-Pack for System Builders - OEM
Antec Two Hundred Black ATX Mid Tower Computer Case - Retail
Model #:Two Hundred
$54.99 -$15.00 Instant $39.99
G.SKILL Ripjaws Series 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-4GBRL - Retail
I did not search for combos, so saving a few bucks is still likely possible. Still, with shipping, this might break your budget. Of course, if you don't need the OS, you're good. If you do, consider temporarily skipping the video card, or pick up a HD4650 for <$60. The IGP on the 790GX board is good enough for Sims 3 and all your friend's other apps, especially at only 1280x1024; I wouldn't be surprised if it can handle Diablo 3 as well. When you get your friend into more demanding games, or at the time of a monitor upgrade, you can add a better video card. The 380W Antec Earthwatts can easily handle a HD5750, and probably a HD5770 as well.
If I wanted to add more future-resistance (future "proofing" is an expensive myth), about the only thing I'd change to the build above is I'd get a beefier PSU, perhaps a 550W Antec Truepower New. I'd be confident of being able to add any single video card I'd likely want. You could also upgrade the CPU later too.