As a matter of fact, I bought a GigaDesigns Dual 1.33GHz processor upgrade from OWC (
http://www.macsales.com), to put in my Quicksilver '01.
I had some troubles with the installation (I think I got the wrong heat sink), but GigaDesign's Support person was SUPER helpful and after I thought the upgrade was burned out, I sent it to them and they tested it for me and sent it back to me. I had no troubles the second time around and after more then a year, my Mac is still super fast. I really like the GigaDesign CPU upgrade.
I have purchased a lot of Sonnet CPU upgrade over the years and had troubles with a lot of them. Maybe 25% of them.. and we're talking a total of maybe 20 CPU upgrades and 10 ATA or USB/Firewire cards that I've ordered from them over the last 6 years.
I've never used any of the OWC Mercury products but have heard good things about them.
And finally, OWC rules. I've upgraded my Quicksilver to the max thru them for a decent price. DVR-108, ATA6 card, couple new HDs, some external HDs, CPU upgrade, and video card upgrade. Good prices (for Mac stuff), super fast shipping, and fantastic service and support.