and have nice airflow in my Haf 912.
For a 912 you mean.....it is somewhat light for a high end gaming box......which might have 8 - 10 fans helping ...... here's a way to tell if ya case is in any way affecting your temps.... take of the side door and use something like this
http://www.sears.com/shc/s/p_10153_12605_03275000000P?sid=IDx20070921x00003h&srccode=cii_5784816&cpncode=26-68847505-2
.... to blow huge amounts of air inside ya case..... if the temp doesn't change, then nothing you are going to do case wise is going to help.
However, one 560 Ti shouldn't be having that much of an impact, assuming of course any optional fan mounts have been filled with a decent fan. You have 4 optional fan mounts in that case and if not filled, I would do just that.
http://www.coolermaster-usa.com/landing/haf912/home.php?page=cooling
For comparison's sake, my son's box (Antex DF-85) has 8 fans, CPU is OC'd to 4.6 Ghz (down from 4.8 Ghz for 1st 6 months) and twin 560 Ti's that I just dropped from 1020 Hz to 980.....temps on the cards were 84ish on top card (80 on bottom) so I dropped it down.... all fans are on lowest speed setting. CPU temps are in mid 60's.....these are measured using OCCT's CPU and GPU torture tests.
Of course blowing some compressed air thru the fa shroud is one way to get out the dust.