I upgraded to a ATI Radeon HD4850

What does the rest of your system look like? If your slowest part was your CPU and not your GPU you wouldnt notice a difference, also what graphics card did you upgrade from?

depending on the generation of Athlon's it came from it is anywhere from a 130nm to a 65nm processor, however it is a single core at only 2 or 2.2GHz so that is likely your limiting factor in your system and cant pump out enough to your graphics card, see if you can increase the AA or AF and not take a performance hit, if you can it is your processor. If you have the budget to you may want to build a new system as you already have the graphics card which is a good chunk of the cost, $700 should get you a pretty good system.

Unfortunately being a socket 754 there are no processors currently for sale on newegg that will fit your motherboard, it was phased out in favor of the AM2 socket back in 2006 so i dont think there are any new processors that will fit your motherboard, also a dual or quad core would treat you far nicer than an upgraded single core.

Can you help me find a new MotherBoard that will work with my new video card ?? that has a dual or quad core processor? LOL Please??

Post in the new homebuilt systems forum, as you have a microATX board your case likely wont provide sufficient cooling nor will your power supply provide sufficient power. Follow the sticky at the top of the page and someone should be able to get you a nice set of parts picked out.