Well when you run games or benchmarks, do you get artifacts? If not, then you should be fine. If you run a game/benchmark for a while and it doesn't lock up, then you probably don't need to worry about altering the cooling or anything like that.
My msi card runs at 315/580 still with stock cooling.
W/ out extra coling I only O/C'ed my card for testing.
When I added extra cooling, my o/c stays at 300/600.
Consider extra cooling as protecting your investment.
It does not cost much to add a extra 80mm fan or ram sinks. There are loads of fun toys that will help keep your agp slot cool..or make one yourself. Stockcooling is good on most of those Ti4200's but your case may be warm and high o/c will only incease the temps and your card may get grumpy(artifacts or lockups....even poor performance).