Omega gives you the power to turn up your card to it's fullest. But it can also be dangerous as you can cook your GPU. Unless you want a few more FPS at the possible expence of your card use the ATI drivers, I have used both and saw only 2-4 FPS increase by OCing the piss out of my card and I almost cooked it. I use ATI's drivers as I don't feel the need to OC anymore than ATI will allow.
There is one other side benefit, some cards can be unlocked by using the Omega drivers.
Your 9550 card *most likely* has a 64 bit memory interface. You will get better performance by using a 128Mb interface card. Or a 256.