get a aftermarket cooler (i don' live in America so I donno prices), but do some reviews, I'm sure any goodish looking aftermarket cooler will be good enough to do some decent overclocking if your on a budget.
get RAM first if you need it, cause no overclocking will help you if you don't have enough RAM, if your running 32bit just get 1GB, cause your computer most likely won't recognize more than 3.2GB
You can check RAM usage in task manager (ctrl+alt+del) under performance tab (PF Usage) incase you didn't know how to check RAM usage (cause alota ppl don't)
Running Raid0 will help with boot times and loading times, but you definitely need a backup, cause if 1 drive fails you lose everything (its smart to have a backup anyway).
I'd say go with the 3rd option seeing as you get a aftermarket cooler, some ram and more HDD space (if its in your budget), mind you the 500GB hard drive won't improve much performance if or if any depending on how you set it up.