First real attempt to overclock...

...so advice is appreciated. Here are the details, including temps:

Antec Sonata III with 500w PS, front & back 120mm fans
EVGA 1220-CK-NF68-A1 (nVidia 680i)
e6850 @ 3510MHz @ 9x390 & 1.3125V (stock is 9x333 & 1.2750)
Arctic Cooling Freezer 7 Pro
2GB (2x1GB) Corsair TWIN2X2048-6400C5DHX (5,5,5,18)
2 x BFG 7900GS, SLI
WD Raptor 74GB

Temps under load (Prime95 x 2), give or take, with side panel of Sonata III removed:
cpu/tcase: 62C
core0: 59C
core1: 59C
Temps under load (Prime95 x 2), give or take, with side panel of Sonata III in place:
cpu/tcase: 67C
core0: 64C
core1: 62C
Idle Temps, give or take, with side panel of Sonata III removed:
cpu/tcase: 50C
core0: 47C
core1: 46C
Idle Temps, give or take, with side panel of Sonata III in place:
cpu/tcase: 54C
core0: 51C
core1: 51C
Room Temp: about 24C

All temps rise a few more degrees when I go to full load using TAT. With the case closed both core temps and tcase quickly hit the low 70s.

Doing nothing but FSB to 1560 & upping Vcore to 1.3125, 3510MHz (9x390) seems nice and smooth. Lower Vcore, and Prime95 fails: the lower the voltage (below 1.3125), the faster it fails. At 1.3000 there's a failure within an hour or so; at 1.30625, the failure occurs after 6 or 7 hours.

What I don't really get is the fact that tcase is consistently higher, by a few degrees C, than the cores. (Initially, with the stock Intel HSF, things were worse; the Freezer 7 lowered all temps by maybe 6 degrees.)

Right now, I’m at about 20 hours with Prime95 running on both cores, problem free.

So, I'm wondering:
1) Should I be worried about the temps, especially with the Sonata III panel in place? I know this case wasn't a great choice in many ways, but I'm reluctant to change it now. I figure I can probably just monitor temps, as I'm now in the habit of doing, and remove the panel if things get heated, which probably won't happen too often, given my typical usage. If the above OC & corresponding temps will really reduce CPU life, I can consider better cooling or a new case. Or I can just go back to stock, though temps have always been somewhat high, under TAT load especially, with cores 0/1 hitting 70/70 or so in no time at all.
2) Is having CPU temp higher than core temps just an anomaly, based on my particular e6850, since the relative numbers have never changed, regardless of stock HSF or Freezer 7, or side panel on or off?

Thanks.