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My current Xeon 5650 has to be my best ever CPU. Only £69 for 4GHZ hex-core goodness (even better that it is in a free GA-EX58-UD5/Coolermaster Cosmos which I was given!). I can't see me needing to upgrade for years.
After that i'd never have another AMD CPU again (not like they can even compete with Intel anyway).
PCIe usb 3.0 card will solve that one issue, and they are reliable if you get the right chipset. SATA bandwidth is hardly noticeable to be honest and plenty fast enough, also with SATA 2 i consistently saw speeds in the 250-280mb/sec range, not 200. Not clued up on graphics well enough to say whether PCIe 3.0 is a dealbreaker.
200-250MB/sec is over times less what the new PCI-E SSD's can do.
Also, using addin card controllers to add USB3 etc is never ever as good as natively having it on your chipset. Speeds and reliability will suffer, as well as cluttering your PC up inside.
AMD Athlon 3200+ - Threw everything i had at it in the day and was a stable and reliable CPU for my first ever gaming build (had build previously but were adapted or mixed usage)
But my top one has to be:
I7 920 DO Stepping - Current CPU since Dec 2009 i think, and still bossing everything I throw at it and I still havent even bothered to ramp it upto 4ghz (which it manages easily and at very low voltage). Rock steady, great value for money and been a pleasure to own.
I'm starting to think with the way i use pcs lately i could've kept my 920 and still be happy, did 4.4Ghz with ease and with a full 12GB RAM installed too.