"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 (cost used from summer 2009) and installed them in my Gen1 Mac Pro. (Ref: Intel X5355 CPU info.)
It was a drop in replacement - no software reinstall or compatibility problems of any kind. CPUs are humming along at 91°F (idle) and get up to about 130° 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 with EFI64, PCIe 2.0 slots, faster ram, etc. (FYI: Sometimes you can find used/tested Xeon CPUs (pulls) very cheap. Some reports in 2013 noted $60 or less a pair/used.)
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 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 early intel-based iMac and mini models had socketed CPUs also but later models don't.)

"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
• Use Apple headphones with your iPhone, iPad, and iPod
• Linking your Nike + iPod Sensor to your iPod touch (2nd gen) or iPhone 3GS
• Find the serial number or IMEI on your iPhone, iPad, or iPod touch
• 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