"Gen1 Mac Pro with 2 x Xeon 5355s (8-cores)
I purchased 2 Xeon 5355s (2.66Ghz/8MB/1333/Quad-core) off of eBay for $300 each (bought used he later said), and installed them in my Gen1 Mac Pro. (And he late wrote he sold his used 5150's for $212 for the pair to offset the cost of the 5355s.)
It was a drop in replacement - no software reinstall or compatibility problems of any kind. CPUs are humming along at 91 degrees F (idle) and get up to about 130 degrees maximum when pushed VERY hard with Handbrake. Mac Pro fans and heatsinks were apparently already made for these, as the Pro has no problem keeping them cool.

Speed increase is just about linear with Handbrake versus the 2.66Ghz dual-core 5150s they replaced. Sunspider benchmark on Safari 4 / 10.6 beta with 8 cores is 400 ms (557 on Leopard)! Skype 640x480 video is 5-6% CPU utilization under 10.6. HandBrake h.264 conversions are smokin at 175 fps.

Upgrade results - 9 out of 10, only because the CPUs are not natively recognized in 10.5.7 or 10.6 beta. But I found a CPU utility (AboutThisMac) that fixes that issue nicely.
-John P."

BTW - I had someone write about doing this CPU Upgrade + 32GB of ram in his 2006 Mac Pro - I suggested he total up the cost of all upgrades and consider what the Mac Pro may bring used to see what the total $ (from the sale + what would be spent on upgrades) would buy as far as a later model (new, used or refurb) Mac Pro w/EFI64, PCIe 2.0 slots, faster ram, etc.)
Back in August 2006 I was sent some screenshots (from a hdw developer) on a 1st gen Mac Pro with swapped in 'Clovertown' Quad-Core CPUs (worked fine/PNP) but that was before the CPUs were officially released and I didn't post them at his request. (I still have the folder of those screenshots here.)
[FYI Several readers asked about a guide to upgrading their 1st Gen Mac Pro's CPUs - here's a link to a Youtube Video guide on Processor swaps in an early an Mac Pro (MacWelt TV - in german) and in mid-Sept. 2006 Anandtech posted an article on Upgrading the (1st gen) Mac Pro's CPUs. You'll want a 3mm hex wrench to remove the heatsinks - John said he used an Elkind 3mm hex wrench (9-10" long "T" wrench) he bought for appx $4 at ACE Hardware.]
And back in early Nov 2006 (still linked on Macs/systems page here) I posted a link to Cnet's article with some performance comparison of an 1st gen Mac Pro with original dual-core CPUs vs Quad core Xeon 5355 CPUs. (Socketed CPUs are a big plus - the intel-based iMac and mini models have had socketed CPUs also, but the 2009 Mini no longer does apparently.)

"VirtualBox 3.0.2 (released 2009-07-10) Changelog:
This is a maintenance release. The following items were fixed and/or added:

VMM: fixed network regressions (guest hangs during network IO)

VMM: guest SMP performance improvements

VMM: fixed hangs and poor performance with Kaspersky Internet Security (VT-x/AMD-V only)

VMM: fixed crashes when executing certain Linux guests (software virtualization only)

ACPI: fixed Windows 2000 kernel hangs with IO-APIC enabled

APIC: fixed high idle load for certain Linux guests (3.0 regression)

BIOS: properly handle Ctrl-Alt-Del in real mode

iSCSI: fixed configuration parsing

OVF: fix potential confusion when exporting networks

OVF: compatibility fix

NAT: fixed crashes under certain circumstances

3D support: fixed dynamic linking on Solaris/OpenSolaris guests

3D support: fixed incorrect context/window tracking for multithreaded apps

Shared Folders: fixed loading from saved state

Shared Folders: host file permissions set to 0400 with Windows guest

X11 host and guest clipboard: fixed a number of issues, including bug #4380 and #4344

X11 Additions: fixed some issues with seamless windows in X11 guests

Windows Additions: added VBoxServiceNT for NT4 guests (for time synchronization and guest properties)

Windows Additions: fixed version lookup

Linux hosts: workaround for buggy graphics drivers showing a black VM window on recent distributions

Linux hosts: fixed typo in kernel module startup script

Installer: support Pardus Linux

Solaris hosts: several installer fixes

Solaris host: fixed a preemption issue causing VMs to never start on Solaris 10

Solaris guest: fixed mouse integration for OpenSolaris 2009.06

Windows hosts: fixed high CPU usage after resuming the host

OVF: accept ovf:/disk/ specifiers with a single slash in addition to ovf://disk/

Fixed a settings file conversion bug which sometimes caused hardware acceleration to be enabled for virtual machines that had no explicit configuration in the XML.

I've added a FYI/link on v3.0.2 to the previous page of Mac owner reports on VirtualBox.

• Connecting your Mac to your mobile phone with Bluetooth
• MobileMe, iPhone OS 3.0: Troubleshooting Find My iPhone and Remote Wipe
• Compatibility of Apple wired headset models with iPod and iPhone models
• Linking your Nike + iPod Sensor to your iPod touch (2nd gen) or iPhone 3GS
• iPhone: Locating the serial number, IMEI, and ICCID number
• iMovie '09: Motion JPEG clips cannot be imported from some hybrid cameras
• What to do if "Allow Bluetooth devices to wake this computer" option is not available
• iPhone and iPod touch: About backups
• Keeping your MobileMe account secure

• Apple White MacBook 13in (MC240B/A) Review (trustedreviews)
• Video: Playing with App Expose in Snow Leopard (crunchgear)
• iPhone App Store 1 Year Later: Apple's Highs and Lows (PCWorld)
• ATT Denies Rumors of MMS Delays, Tethering Costs for iPhone (ref Apple iPhone MMS Delayed, Coming Sept, Tethering $55 Extra)
• Time Machine backups to Windows Home Server
• Is Apple trading SD cards for DVDs in MacBooks? (computerworld)
• Video Review: iPhone 3GS (v3 uk)
• Apple Files For More iPhone Patents (informationweek)
  
  Software related/Updates:
• Fetch 5.5.1
• Senuti 1.1.2 (Copy music from iPods to Macs)
• SilverFast Ai Nikon Scan 6.6.1r1a
• Vidalin 1.1 (free Apple TV utility adds titles, desc., etc to your shows)
  
  Other/Misc:
• DirectX targeted in (next tuesday's) Microsoft security updates
• Sharp introduces new LED-backlit LCDs (TVs)
• Left4Dead 2 Interview