I agree with the above poster. Being a 5-boxer myself and having gone through all the hardware upgrades and knowing a little something about computers, your path is this:

You're in a complicated spot. First of all I completely agree with algol in that your number 1 priority needs to be to get more RAM. Quad Core is nice, but based upon what you have, memory is the thing you should do first. In order to get more memory, you want to move to a 64-bit OS. I recommend Vista, but I have used both XP and Vista in 64-bit and they will both work for you. I am going to make an assumption that your processor is a 45nm processor. As long as you'll be moving to 8 GB of RAM instead of 4 GB, I would strongly suggest getting a new motherboard that supports DDR3 to coincide with your memory increase and OS upgrade. You need to be very careful when doing this to make sure you're not losing compatibility with your processor or other components. If your processor would not be supported in a motherboard that supports DDR3, then you have to decide if you want to spend money now on DDR2 memory for your existing motherboard that will probably be phased out before too much longer or if you want to pony up the extra cash for a new processor to coincide with the other upgrades.

For sake of argument, I will assume you go with a DDR3 memory capable motherboard and a 64-bit OS to have 8 GB of RAM and are keeping your old processor. At this point in time, I think you'll be in pretty good shape. You'll find play a lot better and for all practical purposes, your processor will most likely suit you fine (I was doing just fine w/ a Dual Core 2.4 GHz processor running a 5 box). As algol said, again, I think you should turn your attention your video card and not your processor as your next consideration... Although this is said without seeing your system in action. Here's how to tell if it would really make the difference... Is your processor pegged when you are at your worst performance? I'm betting the answer is no... Second, what's your FPS like when you have worst performance? If you find that while the processor isn't completely pegged all the way, your FPS is suffering, then a stronger video card is the way to go. Another thing to consider is that a stronger video card will also off-load your main processor. I just went from a Geforce 7800 GT to a Geforce 285 GTX. I haven't yet tested it out with my 5-box setup, but look forward to doing so either tonight or later this week (depending upon guild raid schedule).

You asked if hard drives would help.... plain and simply... no. I could give a detailed response, but there's not much reason to elaborate. :thumbsup: