This is my rig I bought very recently to replace my old one.

http://www.gateway.com/systems/product/529668208.php

^ Check link for system specs ^

It sports a single Radeon HD 4850 512 MB. If I were to upgrade the power supply to sufficiently power a single Radeon HD 4870 single core or X2, would the computers CPU and other components not have the processing bandwith to handle the cards colossal throughput? Would I be forced to upgrade to a high-clocked Phenom 2 or Core i7 if I wanted to use today's most powerful GPU's? Could I run Crysis on all very high settings at 800X600 or 1024X768 resolution with no major FPS loss using the HD 4870 X2?

Based on the system's specs, what power supply would you recommend for me if I were to upgrade to power the best cards?