As pointed out earlier, you need a full upgrade. There are two routes you can take with that.
1) Upgrade the core (CPU, CPU Cooler, mobo, RAM) first. Your initial budget definetely allows that. It wont be top level but pretty close.
2) Upgrade peripherals (videocard, storage, case) first. I wouldnt suggest it because your mobo doesnt support SATAIII. It likely wont be noticable on your HDD but will definetely be noticable when using SSD.
If you want to go on with the building answering the questions for us from
this thread will be very helpful.