What do you use it for? Overclocking is unneccesary i agree with the other two no, i do wnat to point out that overclocking can destroy your system and the speed advantage is minimal, if you want more speed either buy another 512 Ram or a faster CPU is needed.
If you are into games I would have thought that 1 Gig was p[lenty enough memory. I'm not a gamer but I think you will find one of the most important pieces of hardware is the graphics card. Get the best you can afford.
One of the main problems with overclocking apart from stability of the OS, is that it causes the CPU to get hotter as well, so you need a very good CPU fan at least and preferably a good fan/extraction system in your case to keep tempertures as low as possiable (over heating the CPU with at least shorten it's life, at worst kill it and your mobo and memory)
I would go with what Torwalker has suggeted, get the best graphic card you can afford, this will take some of the pressure off the CPU, but remember that some newer mid to high range cards need there own power souce, so check that you have a spare plug on your PSU and that it is also powerful enough to take the extra power to run the card.