MkGriff1492 :
450D is an okay case. Filters are a nice feature but have to be maintained to work. The second 140mm should have more open airflow. Front fans are nice but not really usefull if there being restricted by the frame. I recommand getting a case you can mount fans to the front of case and back of hdd cages for good airflow. Its really tough to find a good air cooling mid tower. I recommend finding one on the bigger end of midtowers. More room equates to more airflow. If you get the 450D I would get a pci slot cooler to move air under your graphic card.
This is not entirely accurate. I read multiple reviews, and the thermal performance of the 450D is actually pretty good. My temperatures with a 3570K @ 4.3ghz and a GTX 770 SC are fine.
The 450D is actually a better performer than the 750D according to this article, in part because the 450D changed the front to a mesh with dust filter, so it runs slightly cooler and slightly louder.
http://techreport.com/review/26219/corsair-obsidian-series-450d-enclosure-reviewed/4
When it comes to cases, bigger does necessarily mean better. A smaller case with better airflow will outperform a huge case with hot spots...