One of the considerations with RAM is the type of operating system, a 32-bit Windows system will only see between 3
gb and 3.5GB of RAM, so installing 4GB that the motherboard may be able to handle would be too much, and may actually slow down the computer. A 64-bit Windows operating system will be able to us that full amount of RAM.
At this time you won't find a computer that is going to be impervious to extensive loads, they all will slow a little with each application that you add, each web page you open, movies that you watch...
As for bottlenecks, once again I'm going to defer to an article which explains this, there is a lot of information. You can read it here
Another consideration is the speed of the hard drive, the old PATA had a transfer rate of about 100MBps, and the newer SATA drives have a transfer rate of 3Gbps. This is a big difference.
Edit: I did a little looking around and found another article which puts this information in terms that may be easier for you to understand, you can read it here
, I hope this helps.
Edited by dc3, 09 June 2008 - 10:55 AM.