I upgraded my CPU earlier this year after assuring myself that the next gen console ports would be unlikely to require anything above an i7. This has meant little to my current batch of games as they all did fine with my E8400 (though GTA IV finally runs at 60 fps).
The CPU upgrade also meant a mobo and a RAM upgrade as below. Luckily, I managed to get a decent price on my old CPU+MB+RAM.
I picked up an SSD too, just to see what all the fuss was about, and slapped Windows 8 64 bit on there for good measure. Windows 8 is great and the SSD makes a huge difference in general system performance. Loading times are also improved, though that might just be the extra/faster system RAM.
Otherwise, I still plan on waiting for the benchmarks on the first round of next gen console ports before I commit to a GPU upgrade.
Zotac GTX 275, 896 MB GDDR3
Intel i7 3770K
G.SKILL (Ares) 8 GB DDR3 RAM
120 GB Samsung 840 SSD SATA III (with Windows 8 64 bit)
320 GB WD Internal HD SATA (with Windows XP Pro SP3)
320 GB WD Internal HD EIDE (with Windows Vista Enterprise)
2 TB Samsung Internal HD SATA
500 GB Seagate External HD