I guess you are not in a hurry, or you wouldn't be asking the question. It's worth waiting IMO, but only if you have a large budget and you're serious about not doing any upgrades to CPU or GPU for 3 years and you really need lots of processing power.
I'm waiting for a 3.16 GHz 45nm quad CPU and a 9800 GTX. On the other hand, just these two together will probably cost $2000, which means most people won't bother with them.
They are confirmed, as in "VPs from Intel and nVidia have made promises to investors in interviews". There's no 100% guarantee, but they might actually deliver on schedule. It's not like the ATI/AMD case where the merger has created chaos and schedules were blown.
A Q6600 will last you 3 years all right. An 8800 GTX should last you 3 years, unless you have a large monitor (1920x1200 or more) and you want to play Crysis or other demanding games with maximum eye-candy). Right now there is no GPU capable of handling DX10 properly, that's the main reason why I'll wait for the 9800 GTX. Even the 8800 GTX gets as low as 10 fps in some DX10 games or demos at higher resolutions, yuck...
Performance comparisons:
- the 3.16 GHz quad Penryn will have 31.6% speed advantage over the Q6600 thanks to frequency alone.
- add 2% to 6% thanks to the higher FSB (Anandtech compared QX6850 & QX6700 and got these numbers)
- improved floating point unit (may impact some applications a lot, e.g. CAD or Flight Simulator, others not at all)
- lots of new SSE instructions (may speed up video encoding by 40% with the right software)
- better power management, less power consumption, less heat, less noise from fans (at the same clock, that is)
- it will overclock better
Whether this is worth waiting 6 months and paying about $700 more, you decide for yourself.