There aren't any fixed numbers because the overclock varies from card to card. 9550s have the exact same GPU as the 9600 and 9600 Pros. Most 9600s don't have a fan and have a heatsink about the size of a 9550s. 9550s can almost always run the GPU at 9600 speeds (325MHz) without problems. The RAM varies depending on what kind of RAM is on the card but you have to be careful overclocking RAM by very much (more than 5%) because it's easy to permanently damage it. Excessive RAM overclocking can work for a while and then RAM deterioration requires lower and lower clock rates for the RAM to work without artifacts. You can find lots of used video cards on eBay with overclocking-damaged RAM which can't run at stock clock rates anymore. I'd just run ATITool's
artifact test and back off some on GPU clock rate which gives no artifacts. I'd try GPU=325MHz and RAM=425MHz but keep in mind you can damage the video card when overclocking and there's no way to know for sure when you're close to hurting anything. By the way, my Radeon 9700 Pro (GPU=325MHz and RAM=310MHz) is actually a Radeon 9700 (GPU=277MHz and RAM=270MHz) and I haven't had any reliability problems but I upgraded my GPU heatsink and keep the inside of my machine very cool.