I'm sure you're referring to inFamous.
Definitely can't be Infamous. I finished that game and while very cool (underrated imho), it's not in the same visual league as GTA4. It does provide a clue though to some of the performance problems that RDR might be facing on PS3. Let's break it down:
1) Infamous is PS3 only so we can assume that they coded it perfectly since they aren't dumb multi platform devs. So we know they will use spu's, get Sony's help, won't leverage edram, etc, basically they will make optimal use of the PS3 platform.
2) If anyone recalls from playing that game, on the second island there is an old shanty town type area that has more detail that most of the other areas in that game. In that area the framerate takes a hit, it has trouble maintaining 30fps. Looking at RDR, from what I read on the net the framerate takes a hit in the detailed city areas as well on the PS3 version.
3) Looking at Infamous again, they do all manner of cheating to minimize what is drawn. Buildings are laid out not for look, but for culling to block as much as possible. When that doesn't work though one can see really bad lod popping that they try to mitigate with extreme depth of field blurring. RDR on the other hand went for huge viewing distances with less lod pop and less depth of field blurring.
So it seems like there is a pattern here irregardless of it being a PS3 only dev or a multi platform dev. Both of these open world games have frame rate problems on PS3 when detailed areas need to be rendered, but they took different approaches to alleviate it. Infamous didn't drop the resolution and instead decided to live with less impressive visuals, 1/4 resolution un-smoothed transparencies, heavy aliasing and aggressive draw distance culling to keep frame rate at 30fps, and they were willing to live with frame rate issues in that shanty town area. RDR on the other hand went with better visuals, less lod pop, longer draw distance, less depth of field blurring, and less aggressive culling, but dropped the resolution presumably to compensate and keep the frame rate steady, although just like Infamous it also drops frames in the more detailed areas.
So two different approaches. Perhaps RDR on PS3 could have just done what Infamous did, which we know to be well coded since they are PS3 only devs and hence it's totally tailored/optimized to the PS3 platform, and employ far more aggressive lod popping, place hills strategically all over the place to cull objects, add aggressive depth of field blurring, don't bother smoothing out transparencies, but bump up the resolution to 1280x720. Would that have been preferred? There probably would still have been frame rate drops in the detailed city areas, just like in Infamous, but then you would have 1280x720 full hd.
I don't see anyone comparing GTA4 with GT5 or Uncharted 2 or GoW3 here.
Well you can't naturally, they aren't comparable. Tons of reasons why, the least of which is that GTA4 has 24hour lighting cycle which obliterates all forms of performance cheats.