This is the best CPU available for around that price:
Intel Core i7-2600 Sandy Bridge 3.4GHz (3.8GHz Turbo Boost) Desktop Processor
But for the compatibility issue, we need to know your model of motherboard. Because if it is an old one, there's a probability that this processor wont support to that motherboard due to socket types and different FSB speeds.
Which processor are you using right now?