I am afraid GTA4 cannot port to the current generation of PSP. It is simply too sophisticated graphically for the PSPs processer to handle. If there is a PSP2 it could port there asuming that PSP2 would have procesing and GPU abilities similar to 360 or PSP

NFS is hit of miss the Most Wanted on PS2 and Xbox along with Underground were pretty good arcade romps. I think though as arcade racers go the Burnout series crushes NFS for sheer fun. No racing game can match the GT series.