Help on Video Memory

I recently purchased a 4870 1GB graphics card from Diamond Multimedia.
Since i use the ATI Tray Tool application, it allows me to view the amount of "FPS, Free system Mem, Free Video Mem, and Free texture memory, etc.
Throughout my 3 hours of gameplay in Fallout 3. I kept noticing that the Free Video Memory remained at 1014MB. As well as the FTP remained at 1780MB, but the System memory usage was the one that was constantly jumping around, or kept changing.
I am concerned about whether or not if there's a defect in my video ram? Or card.
Also, in the Overdrive tab for CCC, whenever i turn my ram clock beyond 945, I start getting pixels on my screen. While i see many other people who has easily got it to 1100Mhz.
I am just afraid that my Video ram was burned out. If someone could give me a definite or reasonable answer, i'd appreciate it.

Specs: Q6600 Quad, Gigabyte mobo, 750 PSU, 2gb patriot ram, 4870 Diamond 1GB vid card.


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