"Upgraded my original 2006 MacPro 1.1 (Dual Xeon 5130 2Ghz processors) with two Xeon L5320 1.86GHz low power Quad-Core processors and also put in an nVidia 8800GT (Flashed PC model) to replace the original nVidia 7300GT.
The only problem was a partially stripped screw on one of the Processor heat sinks. (Some other 2006 Mac Pro owners also mentioned that.-Mike) I used one of those stripped screw extractors, very carefully! and it came out.
Overall the system seems snappier - Fractal reported 60-65 Gigaflops! Xbench actually went down from 171 to 154. (Doesn't use the extra cores of the slower clock speed CPUs) And by the way, the processors ended costing me $150.00 for the pair and the 8800GT was $170.00 U.S. A bargain, I say!
($150 for a pair was cheap (were they used?) - today I searched ebay and the cheapest price I found was $149.99 each (buy it now priced), other L5320 auctions were higher. Recent previous reports here on faster Quad-Core Clovertowns said they paid $300 each for 2.66GHz (in July 2009) and $400 each for 3GHz (Oct. 2009). Although some bought used CPUs and sold their original dual-core CPUs to offset the cost. And one reader bought a used HP server and swapped the CPUs, then re-sold the server with this original Mac Pro CPUs in it.-Mike)

Here are some Cinebench (R10?) results:

Results with Original CPUs
Processor: Intel Xeon E5130 Dual 2.0GHz (dual-cores)
MHz: 2000
Number of CPUs (cores): 4
Operating System : OS X 32 BIT 10.5.6
Graphics Card: NVIDIA GeForce 7300 GT OpenGL Engine
Resolution: 1650x1050
Color Depth: Millions
****************************************************
Rendering (Single CPU): 2185 CB-CPU
Rendering (Multiple CPU): 7183 CB-CPU
Multiprocessor Speedup: 3.29
Shading (OpenGL Standard) : 2814 CB-GFX

Results after L5320 Swap:
Processor: Dual Intel Xeon L5320 (quad-cores)
MHz : 1867
Number of CPUs (cores): 8
Operating System : OS X 32 BIT 10.6.1
Graphics Card: NVIDIA GeForce 8800 GT OpenGL Engine
Resolution: 1650x1050
Color Depth: Millions
****************************************************
Rendering (Single CPU): 2520 CB-CPU
Rendering (Multiple CPU): 14951 CB-CPU
Multiprocessor Speedup: 5.93
Shading (OpenGL Standard) : 4446 CB-GFX

Regards, Robert B."

Encoding tests with (free) Handbrake (free) IMHO is a good test for the benefits of more cores. (It uses a lot of cores, unlike many common apps.)
And here's links to previously posted 1st gen Mac Pro CPU swap guides - a Youtube Video guide on Processor swaps in an early Mac Pro (MacWelt TV - in german) and a Sept. 2006 Anandtech article on Upgrading the (1st gen) Mac Pro's CPUs. Don't forget a long 3mm hex wrench to remove the heatsinks. For readers in the USA, the Elkind 3mm hex wrench (9-10" long "T" wrench) from ACE hardware seems popular (and cheap - $4 reportedly, although like anything prices may vary). For previous reports on 1st Gen Mac Pro Quad-Core upgrades you can search the database, or see these posts.

• 2008 Mac Pro Dual 3.2GHz (X5482) Quad-Core CPU Upgrade
• 2006 Mac Pro w/3GHz Clovertowns (Quad-Core X5365s)
• 2006 Mac Pro Clovertown (Quad core) CPU Swaps Revisited (2.66GHz)
• 2006 Mac Pro w/Dual 3GHz (X5160) Dual-Core CPU Upgrade