We need a lot more info to really be of help to you.
Is your system a custom build or an OEM? What is the specific motherboard on the system? What type or memory does your current system use? Do you have a retail version of Windows you can reuse? What is the wattage of your existing power supply?
Simply from a power supply standpoint you may be limited to something like a Radeon HD 4670. If you have the 'juice' a Radeon HD 4830 would be an excellent choice from both a price and performance standpoint over a 9800gt - and the HD 4830 will most likely OC up to a range of performance similar to the HD 4850.
A new motherboard may require a new version of Windows - and possibly new RAMs. If your current system uses DDR2 you might want this:
X2 7750 Kuma / Biostar 790gx combo: $149
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.172781
That would leave you $100 for an HD 4830.