Memory speed depends greatly on the system in use. Most systems with a Core2/A64 are running DDR2 from 533-800 in the retail market.
A basic rundown(there is not a big improvement with faster memory then the cpu's FSB) for core2 is to match the memory to the cpu FSB. so if the cpu has a FSB of 800 get 400 memory if its a 1066 cpu 533 should be fine, 1333.....667...you get it. If you are running old used memory this is a good way to go. since you will match it closely with the systems it works best with. If you are getting new memory, Fast stuff is often cheaper and going for that is fine. Faster memory will work at lower speeds if needed, but most boards can run fast speeds anyway.
There are some I7 systems and core2/AMD systems with DDR3 hitting the market as well, but if you are running old ram from other systems, you will not have DDR3 just laying around.
Average hard drives today spin at 7200RPMs, but do not think it means they are no faster. PMR(Perpendicular Magnetic Recording) has made drives much faster then they used to be. Drives average numbers like 70-90mb/sec(there are faster drives and slower drives) vs the 40-55 of just a few years ago.
The latest configs today depend on what you want to spend. You can get fairly value systems and also very high end systems for the right price.