I've got an E8400 and I've been looking at Q9550 and Q9650 on ebay. I know I can get about £75 for the E8400. So if I can get either of those quads for £130ish, I'll be alright.
As for the Q9505, I wouldn't have it over my E8400, or maybe I would...I dunno, it's got half the cache and I just wanted something that was potentially double what my E8400 is now