There are many times that I see people say "I think my PC is slow" or "X is slow on my PC", followed by "I want to buy a new, fast PC."
A tool that may help you determine whether or not your computer is fast or slow is "CPU-Z".
It can be downloaded here:www.cpuid.com/softwares/cpu-z.html
(Available for Windows & Apple)[Moderator EDIT: Caution! Read the following threads before installing!]
Ok, so now you've downloaded it, installed it and run it, what do you need to look at?
There are X boxes that are important:
1) CPU Panel - Core Speed
Speed of your CPU.
2) CPU Panel - Bus Speed
This is how fast your CPU can receive data from memory, disks, etc.
3) SPD Panel - Max Bandwidth
This is the speed at which your memory can service requests
Whilst (1) is important, it is really (2) and (3) that are most important to applications such as Lightroom. This is because when image applications are making changes to pictures, they need to read and write memory in large blocks.
How do I use this information? It will tell you what you've bought. Most likely if you buy something today, it will be faster than what you bought n years ago so you don't need to worry.
When you're putting together bits on a website such as Dell's, you'll often be asked to choose how much RAM and what type it is. On the "SPD" panel, you will see a rating something like "PC3-8500" or "PC3-12800". This will be similar to what you have in (3) above. Having a higher number to the right of the "-" is more important than a high number for CPU Mhz - (1) above - because the CPU today is always faster than the RAM. So spending $100 extra on going from 8GB RAM PC3-10600 to 8GB RAM PC3-12800 is a better use of money than spending $100 on going from 2.6GHz to 2.8GHz.
Hope that helps.