Yes, your cpu most likely has a lot more in it. Your corsair is definitely bettter than your kingston, but it is limiting you also (just at a higher fsb). Your power supply is likely contributing to the problem, but this effect is usually subtle and can only be quantified by replacing it with a more capable unit.
Bear in mind that if you do get 512MB sticks it will make it tough to get all the good out of your cpu without modifying the motherboard for increased memory voltage. 2 256s will overclock a lot more readily, but 1GB of ram certainly isn't any wild luxury these days. This is the only real problem faced by P4 overclockers at present, as the chipsets and cpus have a LOT in them, but the ram is very stressed as you exceed 400MHz by any substantial margin. A BH4 would likely be a god-send, but for the moment you are limited to serious vdimm on BH5s or running PC4000 types with their attendant lousy timings (and the ridicoulous price being charged for such sticks).