One last thing. What would be a good temperature for my CPU? Under full load.

I'm not familiar with the X6 processors, but as a rule of thumb your cpu should never go above 60 degrees for normal use, as it will reduce the cpu's lifespan. It's not unusual for temperatures to go a bit higher for benchmarking/stress tests. More than 80 c will result in automatic throttling and damage. The threshold varies depending on the processor.

Also regarding thermal paste: You must reapply thermal paste every single time the heatsink is removed or switched. You must clean off the old gunk before applying new paste. A microfiber cloth dipped in 70% alchohol solution (what you use for cleaning eyeglasses or cds) works great. If you don't clean and replace the paste, your heatsink will not form a good seal with the cpu and it may fry itself.

Under full load running Prime95 or Intel burn Test, I personally prefer my temps to be under 70c, but will be satisfied with 75. Anything over that I don't feel comfortable running with for very long.

As for thermal paste, I wouldn't use any paste that is provided by the cooler manufacturer, I'd rather use Arctic Silver instead.

Well, regarding the lag i do not have that in my new game. I would say its a cpu and ram issue. Probably at some part ingame my cpu speed not fast enough? However i do have slight delay stuttering when i look around once or twice. On this occasion, i run a program that ultilize my third and four cores affinity. While main two cores reserve for skyrim. My setting in game were custom ultra high which i turn off aa, af set to 8x as well as shadow, decals to medium and high distant object fade on 1360 x 768 resolution. Its fine so far and looking great with fxaa plugin, high resolution detail textures mods...

i5 760, 4gb ram
HD Radeon 6950 2GB
620 PSU with 3 fans plus gpu fans and a big fan blowing at my cpu

Really help keeping cpu temp down and gpu temp is at around 62 degree.