Nice CPU but out of stock by most sellers as it has been replaced.
There are usually a few on fleabay though they may well be used so look for a decent price reduction, they'll often come without heatsink/fan too so budget for that too. Take care with these parts as they may have had a hard life but good deals can be had.
Some doof in the states is selling one new for USD375.00 so watch out for scamming!
The replacement Q9400 is USD189 and the tastier Q9550 is USD249. IMHO the Q9550 feels more like a replacement really as times have moved on and 2.83GHz seems more usable than 2.66 particularly as the 9400 only has 6MB cache.
They're both pretty fast and will work in some well priced Motherboards too so you stand to get a lot of bang for your buck. Having said all that I'd probably be looking at i5 if you don't already have a MB or if you can stretch to the Q9550 then look as the base model i7, it'll be much faster and OCs well, if you need that sort of thing.