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Only if you can sell the xtreme chip somewhere for a nice hefty price...
The Q9550 is over £150 more than the Q9450 and if your are just going to run at stock speeds it is only 6% quicker for a 75% price difference so not worth it.
As stated already the Q9450 will be about 10% quicker than your current qx6700 and you ought to be able to get around £200 for it so a Q9450 will be about 15% more money.
As stated already the the q9450 and q9550 are worth it if you are going to overclock them. On the other hand you should have been able to get 3.5ghz from your current chip as normally the only people who buy qx chips, buy them to overclock so the difference still wouldn't be much as the q9450 and q9550 don't clock that much better.
Depending on what you use your computer for, you'd see a bigger gain from buying E8500 which runs at 3.16Ghz in games but you would lose out if you video encode.
However, If I did go to the Q9450 in the future, would my motherboard support it? I've got an Abit IN9 32XMAX which is an nforce 680i chipset board.
Thanks