Well....your motherboard doesn't support dual channel which would probably result in the biggest increase in performance. Besides that, you'll get a performance increase going to the XMS ram, but it'll be anywhere from $225 -$300 depending on if you want the absolute XTRA LOW LATENCY which is the higher priced. All in all it wouldn't be worth it to pay that much money when you more than likely won't see an astronomical increase in performance, especially since you aren't running in dual channel or even capable of running it for that matter. As for the HD, I really don't know. I'm not sure how much the buffer effects system speed.
If you got money to spend and want an extra 1gb of RAM laying around for a possible second computer in the future, go for it, but for the most part you wont see any huge difference compared to the dent in your wallet