Sales and profit are obviously the most important. Sales and profit come from user satisfaction / buzz / good marketing / good reviews.
However remember GTA3 / GTA: VC / GTA: SA were all done in the GTA3 engine (albeit enhanced for VA and then again for SA) and that sales, according to VGChartz increased with each new installment (11.6m -> 14.2m -> 15.4m)
GTA4 will get great sales for sure but critically the costs of making "GTA4: Next Title" and "GTA4: Final Title" will be much less as the engine and a lot of the elements will be in place already. Surely it would be stupid for Rockstar not to at least release a second GTA4 game using their existing GTA4 technology this generation. This will come out at a time when both the 360 / PS3 have a bigger userbase and will almost certainly sell more.
If all of GTA4 downloadable content is released by mid 2009 then nothing is stopping "GTA4: Next Title" from being released at some point between Xmas 2009 -> Xmas 2010 (I am guessing Summer 2010). Thats the game which is going to make the rediculous profits and maybe this is why EA are lining up a purchase now. "GTA4: Next Title" will have a lower development cost and higher sales.







