Diablo 3 will run on anything that's not a netbook or cellphone. It's a whore like that.
BF3 and SKY will favor the Q9550 because they like m0re C0res.
MW3 is intensive on neither the CPU nor GPU.
I don't think you're gonna find a great price on 8400s and 9550s though.
I was right, just looked up ebay for ya, 8400 going for near $100 and 9550 going for near $200. WTFFFFF
Unless you have those processors already, you should actually sell them, LOL and buy yourself a new 2500k or 3570k
		
		
	 
Dominant-firm or duopoly situation between AMD and Intel.  If Intel has a serious edge, it's a near-monopoly situation.  If there were ten different CPU makers for desktops and workstations, you'd see them introduce product at an MSRP and then the prices would drop within a year and before they discontinued production -- as a result of cutthroat competition.  I think INtel has -- for years -- maintained a strict discipline on pricing through some sort of "Intel product distributor" membership or something with a similar moniker.  
But you [and maybe others here] are right.  It would be false economy to -- say -- get an $80 mATX LGA775 motherboard, pay for the processor and DDR2 RAM, even for a "guest" computer.  For nearly the same price, you could get gen-2 socket-1155 (even an I3 core) with DDR3 and a mobo probably in the range of $135.
I suspect, on the pricing, that they hold out for people already in possession of an LGA775 system who don't want to buy a new one or want to avoid the data transfer and software reinstallation they'd have to do on a rig of the latest or newer generation CPU.  Or -- there's surplus inventory in "retail-box," and it hasn't dried up yet.