for me personally, the best SFF gaming(It is actually my media center, but it works great to grab it and go do some gaming.) I had was my SG05(With a 300 watt SFX power supply, that is what those "mATX" power supplies actually are). You can get a decent one up to about 450 watts from FSP.
It is not so small to need a low profile video card, so may be too big for some.
-H55n, no overclocking in this little thing. 300 watt power supply with its fan swapped out(was too loud, be aware it is some 15mm thick or something).
-I5 750 with as much undervolting as it would take and still be 24 hour prime stable(1.04 volts all cores loaded and upto 1.22 for turbo depending on the number of cores).
-4 gigs of corsair
-2 x 1tb hard drives, but removed one in the name of quieter operation.
-GeminiiS with some mods to fit(and a silverstone 120mm fan slightly thinned down to not hit the power supply, used to be a 12mm thick Scythe)
-AMD HD5770. This thing was more then I bargained for(it was more then enough for med-high game settings).
All going full blast(Prime95 + Just Cause 2) did not pass 180 watts at the wall
Before the fan swap, the power supply has no fan grille any more.