You were given horrible advice, imo.
a) Videocards perform only as well as the CPU it is connected to. You CPU, while not archaic, will limit the selection of cards that would be worth the price. A 4890 would not give you the performance that you would be paying for, it is too powerful for your computer. For your current parts, I would recommend at the highest an ATI4850, or Nvidia 250GTS.
b)700-800w PSUs are for computers with multiple videocards, Overclocked CPUs, and 10 harddrives needing power. It was very irresponible for that support staff to recommend such a powerful PSU for your build. Even if you did install a 4890 (a very powerful card) you could very easily get by with a quality 500-550w PSU.
Antec, Corsair, Seasonic, OCZ, PC Cooling&Power, are just a few of the brands that offer quality parts. Buying a cheap PSU puts your whole system at risk, as when cheap power supplies die, they like to take other parts of the computer with them.
c) You came to right place, lots of people here are great about sharing and educating when someone comes asking for help, provided they are willing to learn.
A few things you can tell us to help us make more direct suggestions:
-The resolution of your monitor, this effects how powerful of a card you can utilize.
-Your Budget