512MB should be fine. 350MHz is slow relative to current processors, but the processor speed is not the overwhelming importance it once had. the important factors are:

processor speed - but the gain diminishes as the speed rises. going from 1GHz to 2GHz will not double the overall performance.

memory - same story. if you go from 510Mb to 1024Mb, for example, you won't gain if what you do doesn't touch the extra memory. on the other hand, too few memory is crippling. and the applications become more memory consuming each year.

disk speed - this is one thing in a computer that tends to wear out faster than the other components. you can regain a bit with regular maintenance (defrag, remove unneeded stuff), but not much.

a graphics board will go a long way to reduce load from the processor and memory.