Well, for personal use, i would suggest the Q6600, many performance test prove that there isn't that much different between the lower priced 45nm and the 60nm processors. The q6600 is very, very overclockable and you can do 3.2 ghz with a small upgrade easily, making it as good as the much more expensive ones.
You have the second fasted CPU made for socket A already. Upgrade to that old of a system is not feasible. Consider a total new system, all today's budget builds (plenty of threads on this forum) would be way faster than your current system.