i'm not entirely sure on the "OEM" heatsink, but it sounds like it's not properly seated on the CPU, it would be a good idea to take it off, clean off the old thermalpaste and apply some new thermalpaste and re-attach it.
My Pentium D 930 (With OEM Heatsink) seems very hot. it's about 65C reported by BIOS at idle. But motherboard temperature is just about 42C.
DO i need to buy another heatsink ?
Pentium D 930
Antec Sonata II Case
If it's that hot, the heatsink your using is not seated right in the first place. Once you mount a 775 heatsink, I always tug on it to ensure the feet are properly seated to the board. Otherwise, you'll continue to have heat problems. Another way to solve it is use a cooler from zalman or the like, where you actually screw a plate to the board which IMO, is much better.
No, not at all, the vickers hardness factor of the plastic credit card is way less than that of the aluminium/copper - you can't scratch the heatsink with the credit card. Arctic silver recommends the pea drop and press because that's all that's needed for LGA775 (all the heat is generated in the middle of the heatspreader). Us AMDers have to spread the thermal compound around anyway.
i replaced the stock HSF with coolermaster 48 ( no budget for ASUS VR one , sigh). The idle temp is now around 48C. The heatsink is a bit heavy ! I am using a tower case , i am afraid that it will cause damage to the motherboard.
Well, I think intel says that heatsinks shouldn't be heavier that 600g or something but there are some heatsinks out there (often the best that enthusiasts buy) that are near to 1Kg. Don't worry about it, there's a safety factor built in and I've never heard people reporting problems. If you happen to have a heavy heatsink just be a bit careful when moving your computer around - keep it well secured and avoid shipping it by air at all costs!