Like Carl posted, you have to turn it down to get it to work. The lower, the better, though most people can play single player on high for hours without a crash. Online it's just better for everyone to be set to low when you play. Your specific reason why it's crashing at 1.7 is maybe because you're out of RAM before you hit the 2 gig limit? You can't always base it off of what you're seeing being used by one program you have to add them all up and give a little room for inaccurate reporting of RAM usage.
As for LAA (Large Address Aware), it's been posted time and time again as a fix and that's just bullshit. Awareness does not equal ability. The switch only allows a program to use larger addresses so that it's not in direct competition for RAM that's in use by other programs that do not have it enabled. It, in no way, shape, or form, allows a 32 bit program to use more RAM. 2gigs is the limit for 32bit software. Only a 64 bit program can do that. Now some people like to call themselves software engineers fresh out of school even though they're really just entry level programmers with zero experience in the industry and they think they know better then the hundreds of thousands of programmers that came before them and will try to confuse you with fancy words and link to information about servers rather then personal computers and use words that they learned which aren't the words everyone else uses to confuse the issue, and tell you that RAM doesn't matter because programs don't use it to run which is completely asinine since if that was true then we wouldn't need RAM at all, but hey, the truth is that every time someone brings it up, they explain it wrong, do not include the correct information, or just don't know what the hell they're talking about and when people try it, they come to the proper conclusion and that it just doesn't work as it's being advertised. And as for messing with your 32 bit system settings to get it to work like a 64bit OS and saying it's fine to try is the stupidest thing I've ever heard because all of the other software and hardware settings on your PC can get screwed up sending you into hours of repair to undo the damage you've done by screwing with it after you've found out you were misled in the first place. These test programs to show you that it works are pure bullshit as well because all they're doing is writing and showing you they can write... A true test would be one that uses your sound drivers, uses your video drivers, crunches complex numbers, writes and reads, accesses multiple files at the same time (thousands in the case of Sins), and changes it all up randomly on the fly for hours... You know, all of those things your system has to do in order to play a game... None of them would hold up to the test.
Long story short, if you have a 64bit OS with lots of RAM (4gigs to whatever), you can enable the switch and get better performance since the game can use addresses not in competition with other programs and not in competition with system used RAM, a 32bit OS gains nothing because the system itself will not allow it to use the larger addresses. The game is still only using 2 gigs of RAM though even on a 64 bit system because it was coded as a 32 bit program and thus by the rules of coding in place, it cannot use more then 2.
I should know, I've played the game on both 32 bit and 64 bit systems, 3 and 4 gigs of RAM respectively, LAA enabled and not, and there's no real appreciable amount of game play gained in any scenario regardless of what some random fan of the game might say. They all play the same, they all use the same amounts of RAM, they all crash at the same general times when using the same settings. I've also got more then a decade of experience in the gaming industry as well as modding for fun on the side so I know I'm doing it right giving it the best possible chance to succeed, and not one person I've ever talked to who is still active in the industry about LAA has ever said LAA fixes anything other then tricking the end user into believing things are better. So save yourselves the trouble and just follow the recommendation of turning the settings down for now until TSOP has done everything that they can and us modders have had a chance to incorporate their work into the things we're doing and you don't have to screw with anything potentially damaging your system in the process.
Have a nice day.