(Note: This is concerning the retail edition of the game, not the Steam version)
I use Toca Edit's Widescreen FOV fix tool when I play GTA3. It makes the screen add to the sides, rather than cut off the top and bottom for widescreen. It doesn't edit any DLL files, I think it just temporarily modifies an SCM file.
However, when I'm playing GTA3 with the FOV fix, I cannot go into the overlay without the game crashing. It works fine when it's in the menu, but during the actual game, the overlay causes the game to crash. There is no error message.
Windows 7 Ultimate
AMD Athlon 64 X2 3800+ (2.0GHz)
2GB DDR2 RAM
SAPPHIRE Radeon X1800GTO (256mb VRAM)
Acer X203H Monitor (1600x900)
Draw Distance Max
Frame Limiter On
Frame Sync Off