Im not going to even bother with the quotes, ill just put in my .02:
1)ATITool sometimes overrates your graphics card on its max core/mem. I suggest trying it out and then running 3dmark2003 on troll's lair (texture tearing) and nature (artifacts in water) and scanning for abnormalities. The items in perenthesis are what you should look for, becasue when i oced my 9800p 2 days ago that is what i noticed, and they only appeared within those 2 tests. not even in games.
2) You dont NEED a VGA silencer to oc. It gets u a higher oc but depending on what your cooler is on your 9800p (what board maker made the card), and depending on if you have an r350 or r360 core, u may be able to get it close/at/past 9800xt speeds with the stock cooler. With my r350 core ati 9800p stock (ati made the board), im currently running at 406/358 in an extremely hot room (necessary for stress testing) w/o any visual abnormalities in anything i throw at it.
3) The overclock does help. Halo, farcry and Doom 3 are just 3 games that i have noticed or will notice the benefit of overclocking.
-what i suggest:
bump your core up by 3's and run the tests i told you about (trolls lair, nature) and watch them each a few times. If it runs smooth, bump it up another 3. repeat until you notice something. then back it back down and run it again at 3 less then where u noticed the abnormality. then do the same with memory.
-what to remember:
run it in a warm room. (no ac, no external fans, etc) You want to get the worse case scenario for your card. Dont intentially 150 degrees, but make sure its a good representation of what your room will get up to on a nice hot day.
run other games. Games use different parts of the video card differently. One game i suggest that you must eventually test with your overclock is Doom 3 (after it comes out). This game is rumored to be quite harsh on cards. I suggest also running your other top games for a few hrs just to double check your work.
when initially running 3dmark, run it a few times. This will make sure the card gets nice and hot. I looped the entire demo 5x after i thought i had my stable max overclock. Make sure u watch it closely.