If you are playing at very high resolutions (above 1080p or multiple monitors), or VRAM intensive games (like Skyrim with mods, or Crysis 2 with HD textures), the extra 1 GB or VRAM will let the 7770 overtake the 7790. The 7770 is also factory overclocked with helps it close the gap.
Otherwise, 7790 is better hands down. You get Bioshock free with it.
Thanks. I won't be going above 1080p, but I do plan on picking up Skyrim. Would only having the 1GB have much of an effect on performance?
Would both cards be ok with a 500w power supply?