i7 920 is cheap, but the 1366 motherboard itself will cost you at least 70 - 100 dollars, that would put your total up to 110+.
Since you are on a super low budget, go for the i3 combo deal for 95 dollars and buy a decent GPU.
If the one you have in the system was a PC2-4200, then it does not matter if you buy a PC2-4200 or PC2-5300. However, if it was a PC2-5300, it is recommended to buy another same speed or higher because it will downgrade to that speed automatically.
CPU PLL VCore (1.800) --- > Drop it to 1.5 or 1.6 before you chip die
So the system freezes when you are running Prime95 or Linx? if so, one, it could be the CPU is too hot causes the system to be unstable. Two, you need more vcore. I would keep an eye on the CPU temp while running Prime95...