Thing I find that causes "stutter" lately is something similar to a Raven thread on vsync, some games seem to of late, both Nvidia and ATi, cause a lot of stuttering with vsync on, but vsync off are perfectly smooth.
RUnning xfire myself, in some games I get what you'd call stutter, but I get the same with single card, there will always be the odd game that just sucks on either xfire, or sli, and frankly GTA4 just sucks balls on ANY card around, and both consoles, its just a a truly epic failure of a game in terms of engine/smoothness/optimisation. Night time over the bridges on consoles, the horrible lighting and painfully low fps make my eyes bleed(well not bleed, but hurt like a son of a *****). I've played it in both xfire and single and found very little difference, its smooth in certain area's and a little jerky and a lot slower in others. Dual core 4gb mem single card, quad core at 4Ghz + 8gb mem, xfire, heavily overclocked cards, its still a pile of tripe.
Basically I wouldn't base anything, any opinion about either company off that. Metro 2033, disable the unnoticeable tesselation, the frankly awful DOF and you'll go from very very meh performance on a 480gtx/5870, to completely smooth to be honest. Its a purdy game, with little substance, and tough on hardware even without the two useless options.
Theres more issues with individual games that simply show up with certain settings, than actually run specifically badly on AMD/Nvidia hardware.
In Just Cause 2's case, I have no idea why but without vsync I can stay pretty much above 100fps max out with ease anywhere, I put on vsync (xfire or single) and the framerate is a jerky crappy mess. Raven's had that with GTA4 aswell and I've noticed more and more games with the issue in the past couple years.
The other problem is, peoples eyes vary, screens vary in quality wildly, people setting various options changes and things as simple as one person calling a problem tearing another person calls microstutter and another person calls GTA4 issues.
I still wish, Nvidia, AMD, Intel, Asus, Gigabyte, basically everyone had a "standard" settings page on their websites listing every update, every gaming performance, every setting that is known to work so everyones got a check list of things to do and options that don't work in games to work from.
I think you can take pretty much any gpu setup, Single/sli Nvidia, single/xfire AMD and find settings that make a game work very well, and take any setup and find one setting that makes it run like crap, and thats really the problem. I've been saying it a lot lately, frankly neither company has "driver" issues, just different control panels, different options and different levels of experience, there isn't a game I can't get my AMD hardware running perfectly smoothly, and given time I'm sure the same is true of any Nvidia setup, I haven't used the Nvidia control panel in ages though and if I just set what I think equal my AMD settings I'll probably run into an issue or two and COULD just call it crap Nvidia drivers if I wanted, when infact its more experience and user error than anything else
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