I wouldn't expect to get temperatures under 80c while overclocking to 4.8ghz with the heatsink you are using (even though it is a good one.) I've come to a belief that the only way someone can overclock the i7 cpus at such high levels under 75c is to go with a water cooling system. Air just can't keep up.
This is my set up:
Intel i7 930 cpu
ASUS p6x58d premium motherboard
EVGA gtx 480 1536MG video card
Corsair Dominator DDR3 6GB (3x2g) ram
Antec TruePower 750 watt power supply
Antec 900 case
Corsair Hydro Series H50 CPU cooler
Western Digital Black SATA 1TB hard drive
Asus CD/DVD optical drive
Windows 7 Home Edition
I had my CPU overclocked to 4.0ghz and was getting ~81c on all cores on prime95. I also felt that I would never get those temperatures on any real programs, but after a few months of using my computer I ended up reaching mid 80s on BF2 Badcompany. This is largely due to living in southern AZ where it is starting to heat up and I have no ac. Still, for protection I stopped overclocking and I think I will maybe put it back to around 3.6-3.8.
In game, I can run with maxed settings at around 22fps I think. But if I lower the AA to 8x I think I'm getting around 30fps. I could probably take one more setting down and run the game really smoothly, but I like looking at the crisp textures.
Once the gtx 480 drops in price I may add another one with SLI and get a 1000 watt ps. Though I'm a little uncomfortable about being so energy greedy.