I make a similar post a long time ago, the conclusions that i arrive are:
1)the image "quality" depends a lot of the monitor and the person, for ME and my monitor i see the difference between x4 and x5 (in x5 the textures are a lot clear FOR ME, the aliasing difference is not THAT much, but the texture clarity is noticeable).
*A Jpg image is not point of comparison, the compression of the format make the Imagen lost a lot of detail, if some 1 know a host that not convert to jpg i be glad to upload the shots there.
2) the higher resolution performance is affected by the vram and memory bandwidth (advantage for nvidia over AMD here if u are going to make a buy based on PCSX2 performance), i find my 580GTX with 3gb of ram enough to play at 5x mostly of the time at 60fps, the game have some slowdowns in the beginning and end of the fights but generally in the rest of the scenarios it works pretty ok, except the area near the casino, if u are referring to THAT slowdowns u are going to need IDK maybe a card with 300 GB/s memory bandwidth
*SLI/crossfire setups are not supported by the emu, there is no benefit on it.
so my point here is:
U don't need to make an upgrade with that card because the 680 and the 580 have the same memory bandwidth 192 GB/sec.
*Related question, is there any way to enhance the way that pscx2 is affected by the memory bandwidth of the cards vram?, i don't know how it works, and maybe is a really stupid question, but ey!, if i don't ask im never going to know the answer.
Well, sorry for my bad English it is not my natural language.