So... you're looking to recycle most of the parts but just want to add an AGP/PCI-x16 card so she can play The Sims or something at high res?
If you want a board with good backward and well as foward compatibility, the ASRock P4Dual-915GL is a good board, has both AGP and PCI-x16, so you can toss her whatever old junk card you have laying around and it'll work.
http://www.newegg.com/Product/Product.a ... 6813157075
I don't know if it would be worth it to upgrade such an old system though, or if a standard ATX motherboard would fit into that Dell case. I'm not sure.
Perhaps a PCI graphics card would do the trick? They're fairly inexpensive and do a fairly decent job for The Sims or RTS games and the like.
Maybe a GeForce 5200/5500 or Radeon 9250?
http://www.newegg.com/Product/Product.a ... 6814130255
http://www.newegg.com/Product/Product.a ... 6814140027
http://www.newegg.com/Product/Product.a ... 6814102522
I'd probably go for the 5500 though, best performance of the three, though that's not saying too much.
An interesting comment I found for the 5200 card there:
Comments: I am using a 2.2 Celeron 512DDR, games I am running at decent settings are Call of Duty (high), Farcry (Low), UT2K4 (med), Castle Wolfenstien (high), Warcraft:FT (med/high) just to name a few. If all you have is a pci slot then this card is the one to get =] This card will definetly hold you over till you can upgrade to an AGP board.