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It rarely happens. You might see the Q9550 drop to ~£150 but it probably won't. There won't be that many around now plus everyone gets the same idea you have now.
I was thinking of picking the Q9650 because its the fastest chip so its gonna maintain its value better than the others for selling later. Make sence or not?
Its a dead socket, however some of it's cpus still have very comparable performance to the cpus on the newer platforms. Most people will be looking for quad interim upgrades on this socket so if anything prices will go up in my opinion.
Some new Core2 processors have been launched and there are price drops on others:
http://news.softpedia.com/news/Intel-Perpetuates-the-LGA-775-Processor-Line-132353.shtml
The Q9500 is being launched and seems to be a 6MB version of the Q9550 and hopefully be cheaper.