Which HD 6670 you have, GDDR3 or GDDR5 one? I have the faster one (GDDR5), though GTA IV still lags at maximum settings as well.
For more commandline information, this is a good guide -
http://gtaforums.com/topic/382923-how-to-use-command-line-parameters/When I find my GTA IV directory, I can check, what settings I have there at the moment.
EDIT: Apparently, I had used some commands for shortcut (works same way), however do not have the file anymore. If I recall correctly, I used -nomemrestrict, though -memrestrict with a certain amount of memory set is more stable and recommended.