Does seeing no artefacts on the omega test mean my radeon is stable and CAN withstand the speed ive overclocked it to ?

yes.. BUT:
remember when overclocking the expected lifespan of your card decreases, and you void warrenty.

I dont agree on two counts. You cant run some silly random artifact tester but until you run 3d tests and all your games then you cannot 100% be sure that the card will not artifact. Considering how long a cards useful lifespan is - shortening the workable lifetime by a couple of years means its totally irrelanvant. What the RMA people don't know wont hurt them. The only real danger from overclocking is where you also voltmod. I have heard a nForce2 north bridge running at 2v has a life expectancy of a few months. I would say if you "super" cool your component then this would allow high levels of voltage, since the component is running cooler than designed for.

you wont know till you play a game cause thats when the graphix card actually heats up more you can get 0 artifacts on the tester but if you start to play a game you will see minor artifacts depending on how high you clocked it

ie: i oced my card really high, tester said 0 artifacts

played splinter cell: started getting lines that look like blood ( :o )
played morrowind: started to see some texture breakups ( )
played ghost recon: started to see different colors on the ground leaves were red colours were more redish

basically the program isnt as accurate yet (wait for future releases) its just a simple program to see if you are ocing too high, but the best way is to actually play a game or run a benchmark, thats when the card is actually doing some work

Benchmark ?
You mean a 24 hour 3dmark loop-around ?

I hear thats one way to fully test your card in its new state...