Well, I'll try to give a summary :
Voodoo4 4500 :
- 32MB
- 4x AGP (3dfx reference design only)
- 1x VSA100 @ 166MHz
- max. 2x FSAA
- Fillrate : 333MPixels/sec
Voodoo5 5500 :
- 64MB (32MB per VSA100)
- 2x AGP
- 2x VSA100 @ 166MHz
- max. 4x FSAA
- Fillrate : 667MPixels/sec
- supports T-Buffer cinematic Effects
(Motion Blur, Depth of Field, Soft Shadows, Soft Reflections)
- requires internal Power Connector
- 2nd VSA-100 only active during 3D Display (unless deactivated in 3dfx Tools)
- considerably larger Card
In very rare cases, the Voodoo5's SLI design has lead to compatibility issues with Games, but AFAIK, they were all resolved at some point in time.