ok you could buy this cpu:
the bottom 1 but im not sure how much of an increase if any it would be.
and upgrade ram if possible
think that will work with your motherboard but it depends on if you already have 1 stick of 512mb or are using 2 sticks of 256mb
your motherboard has 2 different ram slots for 2 different pins not sure if you can use 4 rams if 2 are different pins than the others.