4200 rpm hard drive?

Nope the Ram wont make a difference except in dependance of what you're running. In theory it means that applications can use up more ram to store cach data: the less you have the more likely it is the cache will need to be cleared more regularly and data will have to be loaded from the hard drive more frequently..... but in practice this makes little difference at all.

the thing about the HDD performance: 4200 is the slower/slowest end these days but it also tends to run a lot colder and consume less power and is quieter. go to a 5200 or a 7200 or a 9200 RPM'r and then you start to get noisier, more power hungry and hotter - I have a 5200 myself and honestly its damn quiet but theres always a hot spot where my left hand rests (where the HDD is).

If you have the money to upgrade to 4GB ram (you mad thing) I would instead recommend upgrading the HDD speed a bit more and maybe knocking up the processor a grade: anything with a 4MB L2 cache is very reccomendable - the L2 cache has serious performance implications.

No, the hard drive will continue to perform at 4200rpm...there's nothing you can do about that except buy a faster hard drive (and I suggest you do). I upgraded my wife's Thinkpad R51 from a 4200rpm drive to a 7200rpm drive...it makes a HUGE difference.

Nope. I would get a 7200 rpm or a 10000 rpm drive.

No. ram will make your computer open the file faster it has nothing to do with rpm.