I just use radeonator, i have an oem card that wont go above 337/337 speeds without modifying voltages and extreme cooling but good cards with faster memory can go upto 380mhz cpu and 350 (700) mem without serious modding. If your a modder though, i suppose 400-500mhz cpu and 800mhz memory isnt impossible!
I have a Giga-Byte 9700 Pro and so far the max stable settings i can get are 350 Mhz core and 345 Mhz Memory. I got the memory up to 360 but with serious artifacting. I might be able to get it up to 350 but havent tried yet. I am still kinda scared to push up the core more. I plan to get water cooling in a few months so hopefully it can be pushed up more.