If your system is old, perhaps the bearings of the PSU fan are failing. Are you sure that is the source of the noise?
Please provide your system specs. Also, what programs and/or games do you want to run? There are many graphics cards available, from $30 to $300 that will run on a 500W PSU. We need to know what you'd like to run, your budget, and your monitor resolution.
Any modern, quality PSU should run relatively quietly, especially when the system is relatively idle (e.g. at Startup).