Imac I7 Or Quad core tower

Hi guys I have imam i7 that has that screen. Now I am thinking of buying used Mac pro 8 core early 2008 or Mac pro tower quad core Intel xion 3.0 GHz my question is would those 2 towers be as fast as imam i7. When it comes to rendering. I am thinking of replacing my imac since it has load of problems that can happen. I am thining how much of rendering power will i lose?

Thanks guys

What about those spec.
Mac Pro Tower - Intel 2.66 Quad-Core Xeon - (2006/XEON Original Clovertown) (MacPro 1,1)
Model Identifier: MacPro1,1
Processor Name: Dual-Core Intel Xeon
Processor Speed: 2.66 GHz
Number Of Processors: 2
Total Number Of Cores: 4
L2 Cache (per processor): 4 MB
Bus Speed: 1.33 GHz
(2) FireWire 400 Ports (1 Front, 1 Back)
(2) FireWire 800 Ports (1 Front, 1 Back)
(5) USB 2.0 Ports (2 Front, 3 Back)
Optical Audio I/O
Analog Audio I/O

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