Intel's LGA775 stock heatsink and fan isn't all that good, to tell the truth. It's not all that large and the fan gets noisy at higher RPMs, so you either have a quiet system that's hot or a decent-temperature system that's noisy. I work with a custom-built E6700 at work and it has the stock HSF setup on it. The CPU fan spins at about 1800 rpm to keep noise down and the case is Antec's P190 IIRC (yeah, the IT department got a blank check- on this one.) Idle temps are about 43 C and full-load temps are 56-58 C. Athlon 64 X2s excepting the 6000+ run much cooler at similar noise and TDP levels due to a better heatsink design. My 65-watt X2 4200+ on the stock K8 heatsink and 1900 rpm on the fan will run full-load around 39-42 C, depending on how long it's been since I've dusted out the machine. It is true that an Intel CPU with a certain TDP will put off more heat than an AMD CPU with an equivalent TDP, but that's relatively minor and doesn't explain a 25 C difference. My 4200+ running at the standard-version 1.35 Vcore throws off 89 watts and runs around 50 C full-load.
50 C will not harm a C2D, but I'd get a better heatsink if I was you. An idle of 50 C leads to a full-load temp in the 60s, which is getting a little warm. It's a shame that Intel puts a cruddy heatsink on an otherwise very good chip.