Well thats not really a simple question, and it also really depends on the game. Its also comparing to parts of the system which are important and well, its not really a fair comparison.
The cpu is the most important, but a balanced system will get the best results, for exapmle, a high end quad core with 512mb is gonna perform poorly because it doesnt have enough ram. 2GBs of ram is about standard these days. 1GB is too little for most newer games, but ok for games a few years old. 4GBs and above has small, debatable performance gains. Basically 2GB should be fine.
The speed of the ram is also important, as are the timings to a lesser extent. The aim is to get the fastest ram with the lowest timings, within reason. These days standard is DDR2 800Mhz ram with timings of 4-4-4-15.