I just build a new PC this week with a ECS 8800 GTS 512mb and everything set up and ran great.  I wanted to push my vid card a bit to see what i could get out of it and I followed the guide taking just the GPU core frequency up by 10mhz at a time until I started seeing artifacts.  The problem came when I backed the clock rate down.  I eventually ended back up at the factory default settings and I still have artifacts.  My GPU temp never went over 50C.

Hmm, I turned my computer off for about 20 mins and they seem to have gone away so I guess you can disregard this post.

Read the OCing guide here to achieve your highest OC. If you continue getting artifacts at stock speeds or lower, return the vidcard.

I left it sit and they went away and now i have it clocked up from 650mhz to 700mhz and still no artifacts.

I just purchased the same video card. Was able to OC mine to 784Mhz core. Wasn't able to OC the Memory at all though.
 
Using Rivatuner and set up a "Launcher" for my OC. Seems to work fine. Only set my fan up to 75% to keep the noise down just a little. Have played Assassin's Creed the last two nights without issue.
 
LOL Just beat the game too, have to say I'm disappointed with the ending.

I have my memory clocked up to 999mhz