for what purpose? gaming? Since your current motherboard supports AMD processors, just get the fastest phenom II x4 you can afford if its for gaming. If your using it for multithreaded apps, look at the phenom II 6 core varieties.
What denomination of currency are you talking about for your budget? To be honest, when it comes to price vs. performance, Intel has a very hard time matching AMD for anything less than $250 for both the CPU and MB combined.
There are folks on this board who will advise you that anything less than a i5 2500K is not worth it, but you will spend $200 on a i5 2500K alone. For that price, you can get a Phenom II 955 and an AM3 mainboard, while you would still have to spend about $100 more on the i5 to buy a mainboard. Granted, you get about 35-40% improved performance, but you pay for it.