Interested in CPU, power supply, and hard drive upgrade on a stock mother board.?

I have an HP 8200n computer. It's a couple of years old, it originally ran Vista and I've recently upgraded it to Windows 7 32-Bit and also put a Graphics card in it, a NVIDIA GT 430. What would be the smartest thing to start off with? I've been having trouble with the computer randomly freezing... show more I have an HP 8200n computer. It's a couple of years old, it originally ran Vista and I've recently upgraded it to Windows 7 32-Bit and also put a Graphics card in it, a NVIDIA GT 430. What would be the smartest thing to start off with? I've been having trouble with the computer randomly freezing lately as well and I was wondering if it was because I don't have enough power running to run the graphics card and the CPU or something. I'm new at computer upgrades and everything and I'm really interested in fixing this computer up and turning it into a good gaming and all around good PC. So any of your comments would be nice to hear. Thanks!
Update: I'm looking to spend 300-450 dollars on an upgrade build. I understand that the gpu is pretty weak, I needed to get one that would run on my stock psu.
Update 2: Also, I have 4gigs ram and my normal temp for the gpu was anywhere from 38-43c on normal surfing and 62c and lower on gaming
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