Is there a significant difference in performance (especially in a gaming rig) between these two?
My local supplier already has a Q8400, but he would have to order from another company in order to get me a Q9550.
Or would a Q9550 processor an overkill for my gaming rig? (my rig consists of 4gb ram, DG45ID, HD4890, 320gb, yeah my rig sucks but I'm not in the US and no online company ships here...thats the best hardware I can get from him at the moment)
Unfortunately, you need to decide. With a good cooler, it's easy to OC a Q9550 to 3.4 GHz - raise the FSB to 400 MHz. and increase the CPU voltage a little. And you can push most higher. A Q8400 is more work.
I work in Saudi Arabia, so I have some similar problems. CPU prices aren't too bad, but there is limited choice. Motherboard prices are 30 - 50% more than US prices. Upper end graphics cards are 50 - 70% higher.