Graphics card upgrade help

[GeekMom]

My husband was given Assassin's Creed for Christmas. I have a computer that will work, but needs a new graphics card and I'm lost as to what one to buy. Any and all suggestions would be greatly appreciated! The computer has an AMD Athlon 64X2 4200 processor. The motherboard is an Asus M2N68-LA (Narra). I will also need to get another gig of memory, this only has 1 gig. The onboard graphics card is NVIDIA GeForce 6150SE nForce 430. We're not total gamers, the ATI 4870 is a bit out of my price range. I was given a recomendation before but can't remember what it was. I don't know, something in the $100 +/- range?? He also plays FEAR, Far Cry, stuff like that, they seem to do OK on his laptop but the new game doesn't cut it, so I'm wanting to make this desktop a gaming system within our budget.
Thanks!!