A "cheaper but new" PSU could be ultimately dangerous to your system. Read the assorted reviews of cheap PSUs on jonnyguru.com (or hardwaresecrets.com, or hardocp.com, or xbitlabs.com, or... .)
Post your specs if you'd like assistance with getting the right size, but for a single graphics card up to a 4870, a 500W Antec Earthwatts is sufficient. Those are only $70 now on Newegg.