As mentioned above this game was a terrible port to begin with and even fully patched (via steam) the game is poorly optimized.
I had issues with my alienware I7 extreme running 2x 550ti sli and 24 gig ram so i had the works pretty much and was lucky to keep a steady 40 at times all settings maxed out.
Firstly i compared your gfx cards here
http://gpuboss.com/gpus/Radeon-R9-280X-vs-GeForce-GTX-660
from what i see your card should outperform your friends to a point, however notice that the nvidia core is clocked higher, some games gain a significant performance boost with higher core gpu's because these speeds (including video ram) control how fast the data flows between the gpu core and gfx memory, this is where developers and also nvidia /ati need to do a bit of work in there code as combined with the driver / optimization of the game code has serious effect.
Poor / old drivers are one of the largest causes for game performance hence why any tech guy will suggest as no. 1 to update gfx drivers. (so if you haven't done so already go grab em here
http://support.amd.com/en-us/download/auto-detect-tool
One last side note about gfx drivers is the fact that some driver versions are more optimized to a particular game, not sure if you ever read the release notes when drivers are updated ? well if you have you would notice that some drivers will claim upto 40% performance increase or 10% etc etc with certain games, this is because that particular driver has been written to optimize a particular game, so its worth seeing if amd released a driver more suitable for gta iv
You likely already have the game full updated unless you don;t have a home internet connection (hey some people don't lol)
Also to mention here you did not post how much ram you have ? for this game i would suggest easily 6Gb if running windows vista or above x64. Lack of ram most notably with this game 2gb or lower has severe consequences.
If non of the possibilities are to do with old drivers or lack of ram, it may well be pure pot luck in respect to the game, there is one last thing you can try this for more advanced tweaking
http://www.gta4-mods.com/misc/edge-v20-1070-f11749
Anyway i hope this helps some.
all the best Dan
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