Sorry for bumping a 2 year old thread, but multimonitor support does work with GTA SA. Only problem I've had is stretched out words and LOD's are much more visible. One thing you have to make sure to do is if you have bezel compensation enabled, go to WINDOWS display preferences (not AMD's) and select the next resolution (without the compensation). I found that the non-standard resolution (from the bezel fix) causes most games not to recognize that there is an Eyefinity setup, and in most cases most games wont even start at all. Reverting back to a "standard resolution (without the bezel fix) usually fixes that problem. Now when you get in game, you 'll have the option to set the resolution to support eyefinity (You'll also see the resolution with bezel fix there to) just select and your good. as for GTASA, you'll need the ASI file posted on the first page of this thread. I think it's called "GTA SA Widscreen fix lite" or something like that. after that GTASA should be working perfectly. only downside is every time you want to use eyefinity in gta sa, you'll need to switch bezel compensation off.
forgot to mention this: I'm running the game on a tri-monitor setup with 24" screens @1920x1080. so with eyefinity on, my resolution is about 5860x10870 with bezel compensation and 5760x1080 without.