And used RAM rarely comes with a warranty. With so many Macs in common usage, it is difficult to know what kind RAM configuration, type of RAM and RAM speeds your machine requires. They are worth it. Once you've used a 17 inch display, you won't be willing to go back. Sony tends to have the best monitors in this size range.
Larger monitors are even better, but prices go up quickly and the quality range is mixed.
20-inch or larger monitors feature a range of prices directly linked with the quality of the monitor. The best prices are in the back of the various Mac magazines. Stay away from used monitors. After three to four years, video quality deteroriates. This video degradation isn't easy to see unless you have a good monitor next to it.