Personally, i wouldn't buy a new PSU. That one is just fine. Unless you really have to have that new PSU, keep that one the spend that money into an SSD or CPU cooler where you would really see some improvement.
You only need a good quality 600W for that system. The XFX you have currently is more than ample (unless it's over 7ish years old), but if you insist on upgrading I would wait for a deal on newegg etc. Seasonic is the best you can get. XFX, Corsair, and Antec also have PSUs made by Seasonic (note some Antec and Corsair are NOT made by Seasonic, so you will have to do a little more research).
For example I just bought a Seasonic platinum 660W XP2 for 79$ after MIR on newegg. If you aren't in a hurry, be patient and way for the deals. The model I bought is fully modular and comes with a 7 year warranty - don't pay double for the good stuff unless you have to.