Wait until early May. The HD4750 will retail for under $100, play Crysis at 30fps on Medium settings at 1920x1200 (almost HD4850 performance), and be a much better offering than anything in the ~$100 segment today (faster and cooler).
http://www.anandtech.com/guides/showdoc.aspx?i=3538 recommends you wait too.
IMPORTANT: Make sure you current card uses a PCI-Express and not an AGP slot. If it's AGP then almost no current graphics cards will work in it (your best option would be an AGP HD3850 if you can find one). Also, for a system with an AGP slot, your CPU won't be enough to handle Crysis anyway (no single-core can).







