Originally Posted by ///Maluco
It depends on your budget.
1. add some cams or turbo the original motor
2. M30 3.5 big6 swap
3. above with TCD turbo kit (this should get you moving!)
4. something more nostalgic = S14
5. or swap an M88 or S38
Cams don't usully work well in that motor due to the CIS injection system. And if you don't increase the CR, cams are not going to help a great deal.
The cheapest thing to do is probably find an old Callaway turbo kit for the 320i, or drop in an m60!