Yah, GS has it right.
RAM is almost always the best upgrade you can make. Its easy and its very effective. More RAM = More SPEED. Adding even 512 MB of RAM is less than $200, way less than the cost difference between most speed steps of CPUs. If you are talking multiple speed steps, the CPU cost goes way up. Its most effective at lower speeds, because of the relation between the difference in clock speeds to the base clock speed. ie - The 400 MHz difference between a 2.4 and a 2.8 works out to a 16.67% increase, while that same 400 MHz between 2.8 and 3.2 works out to only 14.3%. Meanwhile the cost differential on a 5680 is $35 for the 2.4 to 2.8 while its nearly $400 to go from 2.8 to 3.2, 2.2% vs 24.1%.
On the other hand, the difference between 512 MB RAM and 1024 MB RAM is only $170, a 10% increase.
In the first case, going from 2.4 to 2.8 gets you a 17% increase in speed (which you are not really going to see all of due to other influences) but it only costs you 2.2%, while going from 2.8 to 3.2 gets you a 14.3% increase but at a cost increase of 24.1% and the doubling of the RAM (and you will see most of that impact) costs you 10%.
If you can, try to get a single 512 MB stick instead of two 256 MB sticks. That will allow you to just add another stick instead of tossing the original two 256 MB sticks and getting two new 512 MB sticks. Fact is, if you shop around, you should be able to get good reliable RAM at better prices. Adding RAM is not that difficult.
Hope that helps.