OC capability? 4 GHz is good. You get more, then you're doing way OK
Temps? If we're talking the usually rare max extreme stress, my opinion would be closer to 90° C. Welcome to the forums.
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Intel i7 920 [211] BCLK x 19 = 4.0 GHz @ [1.4000] CPU Voltage & [1.35000] QPI/DRAM Uncore Voltage, Batch 3836A394
3 x 1GB G.SKIL DDR3 1333 (PC3 10666) [DDR3-1691MHz] 10-10-10-24 @ 1.64 DRAM Bus Voltage
ASUS P6T Deluxe v.1 [LGA 1366 Intel X58] BIOS 1102
Thermalright Ultra-120 eXtreme 1366 RT with 120mm Scythe S-Flex F fan
ASUS EAH4850 TOP Radeon HD 4850 512MB @ 680 MHz GPU & 2100 MHz Memory
Antec nine hundred case, two front 120mm fans, one back 120mm Fan, one top 200mm fan
Corsair CMPSU-750TX 750W