It should be safe Mark.
My Gainward 128 meg GF4 Ti4200 is running happily at 310/560 with just the standard cooling.
That said, don't try to do too much too fast.
I wouldn't move the memory up in increments of more than 20mhz at a time up to 500mhz. After each adjustment run the 3dmark benchmark a few times and watch carefully for artifacts, tearing and other graphical anomolies. Also, play some games (both D3D and OpenGL) and watch for the same things. Run the 3dmark demo for at least an hour to make sure your overclock is stable.
Once you reach 500mhz on the memory, start moving it up in increments of 10mhz at a time and run the same safety precautions -- benchmarks, loops, and games.
Do the same thing with the core.
The whole process might take a couple of weeks but it will be time well spent being patient since you probably want to use the card for at least a year.
I'd be suprised if you couldn't run the card at at least 280/520 -- I wouldn't be suprised if you can get even more. But if you set it right out of the box too high you could break the card -- and overclocking will void your warranty.
Good luck.
now that my specs are respectable, I might as well list them again:
2500+ Barton @ 2346mhz, Swiftech MC462a/80mm Sanyo Denki fan
Abit NF7-S v 2.0
2x256 Ocz PC3500 DDR
128 mb ATI Radeon 9800 Pro
Lian-Li PC-68 case
Enermax 431 watt PS
blah blah blah
blah blah blah
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