You might want to sit back for a second and weigh your performance expectations against the hardware's intended use. An Amiga 4000 system with an updated display adpater might pretty up your Workbench and speed up the display of a few things, but you're going to get about the same performance CPU-wise. But if you're thinking about getting a PCI busboard, then why not put one in your 1200 tower?
Trev