Several things are needed for use to help you on this question. First, we need to the know the full specs of your system(CPU, RAM, Motherboard, video, PSU, what resolution your monitor is, and to a degree the storage situation of the computer). Second we need to know your budget.
How to find out the first part of your information..... Well, I suggest going into your "system" page. It will tell you all sorts of stuff, write it down. Your PSU info, many would argue is perhaps the most important info, after your budget, is going to require you do one of 2 things. You can find the model number of your system, and look up what PSU dell stuck in it, on the internet. Or you can pop open your case and do some investigating on the PSU...