It will much depend on how much you want to spend, you may also have to consider how much memory you have and if you need to upgrade both will your PSU be man enough to the task. So a bit to think about...
Thank you Mat Alan - I am willing to upgrade both the video card amd RAM if needed and, of course, assuming it costs more than a new PC. I am surprised you consider PSU a potential problem I had always thought CPU was the main FSX bogie.
You're right about the CPU & FSX, but the main reason the PC is not running FSX at all is because you probably have integrated graphics (GPU), which will only run very basic games (if any games at all).
I would go for something like HD 5670 or GT 240 - about £50-60. They are not very powerful, but will run off the Dell PSU as they don't need external power from the PSU.
You might find the Dell PSU rated at only 250-300W, but they are capable of outputting quite a bit more than that. But if you go for a more powerful GPU than the ones suggested, you'll be looking at upgrading the PSU as well. G
I have just tried AMD's 'Can you run it?'. Their result is that I should not be able to run either FS with my card! Otherwise for CPU rAM etc I got the green bars. Now I go to Amazon and see mingy HD5670 cards - anything wrong with the Sapphires which seem to be at the lower end of the market?