BFG 9800gtx - Paste ok?

first, you need thermal pads, paste + ram chips/mosfets/vrms dont mix at all besides the cooler will not seat right without pads..

second, AS5 is conductive, if you put too much on, youll short the card out.

i reccomend a TIM like OCZ Freeze for these applications, as its not conductive and has great temp results.

those heatsinks are usually just screwed on on the back, take the screws out and it should just pop off.

As PCSarge stated silver conducts.
I prefer Shin Etsu G751 myself, application is also very important.
Try reading this.

They are super easy to get off. They take a while to get all the screws off. To improve temps I would clean off old TIM. Then blow out all the dust out of the heatsink. Reapply TIM of choice (In this case AS5) and game on. Also leave the thermal pads in place.

BTW: AS5 is perfectly fine since you already have it.

Watch out when taking off the cooler since the TIM may put ut a fight since when its cold it acts a bit like glue and you need to twist it to take the cooler off