Haven't posted here for a while but figure I should start again since I just upgraded my rig last week.
The parts I replaced was the following (which I had for about 3 years)
AMD Phenom II X4 955 Black Edition Processor
AMD Asus motherboard
I upgraded to a Intel core I5 2500k Processor, Asus P8Z68-V GEN3 motherboard, and bought some new RAM (8gb) for just under 500 dollars canadian (incl 2 years extended warrantly on the parts for about 40 dollars and incl tax).
the motherboard & the intel processors was part of a bundle and it supports pci-e 3 and I think ivy bridge (though not a big concern right now) with a bios update I think. Think it was about 369.99 cdn before tax for the mother board/cpu bundle.
Only thing I need to upgrade now is the video card since I only got a geforce gtx 560 (non ti) right now and planning to get a 660 (when or if it comes out) or a 680 (in a few months). Also planning to get a SSD drive but not until windows 8 comes out since I really don't want to reinstall windows for a while.
Only thing I can say is that upgrading to the I5 made a big difference in playing games, especially battlefield 3 & skyrym.
You should check out the Windows 8 consumer preview and see if it's worth waiting for. I have it, and I don't plan on switching from 7. You might decide, based on the preview, to go ahead and buy the SSD sooner.
Just a quick update. I did buy a SSD Hard drive for windows today and picked up the Crucial M4 SSD 128gb drive and glad I got the SSD sooner for Windows. Everything runs a lot better when Windows 7 is on a SSD.
If I can make one recommendation for anyone building a new rig or evern upgrading their older rig, get an SSD for your windows intallation and a 1 or 2 TB driver for everthing else.