How often do you see someone upgrading their motherboard and not the processor?
Most of the time, the motherboard upgrade is to accomodate a better processor, so the two go together.
Doing a motherboard alone gives no performance increase, so it is rarely done. Most people want to upgrade their processor, and only upgrade their motherboard when needed. Upgrading to an AM2+ or AM3 setup now just so you can get an AM3+ motherboard later doesn't make a whole lot of sense. Upgrading to an AM2+ or AM3 setup so you can get an AM3+ processor later on down the road does make sense, but again I think the better option there would be to go with the cheaper upgrade solution now that would give you the same performance, then when you want an AM3+ processor, get an AM3+ motherboard.
And for the most part, you are going to see your graphics card holding you back long before your processor. Playing at Max settings is more dependent on your video card than your processor/motherboard. So go with the Q9550, and put the $100+ you save into a better graphics card.
Rig1: System Specs.
Rig2: A8-5600K@4.4GHz / AsRock FM2A75 Pro4 / 8GB Corsair DDR3-1600 9-9-9-24 / HD7560D / Samsung DVD-Burner / 1.5TB WD Green + 3x3TB WD RED in RAID5
Rig3: Athlon X2 4200+ / M4A79 Deluxe / 4GB G.Skill Pi DDR2-800 4-4-4-12 / GT430 / Sony DVD-Burner / 500GB WD
Rig4: Phenom II x6 1605T @ 3.6GHz / Asus M5A99X Evo / 8GB PNY DDR3-1600 9-9-9 / GTX470 & GTX470 / Samsung DVD-Burner / 1.5TB Seagate